feat: add external plugin to build engine

# Conflicts:
#	frontend/cli/cmd_new.go
#	internal/buildengine/deps.go
#	internal/buildengine/engine.go
#	internal/buildengine/languageplugin/go_integration_test.go
#	internal/buildengine/languageplugin/go_plugin_integration_test.go
#	internal/buildengine/languageplugin/go_plugin_tests.go
#	internal/buildengine/languageplugin/plugin.go
#	internal/buildengine/plugin_go_integration_test.go

backend/protos/xyz/block/ftl/v1/language/language.proto

@@ -2,11 +2,87 @@ syntax = "proto3";
 
 package xyz.block.ftl.v1.language;
 
+import "google/protobuf/any.proto";
+import "xyz/block/ftl/v1/ftl.proto";
 import "xyz/block/ftl/v1/schema/schema.proto";
 
 option go_package = "github.com/TBD54566975/ftl/backend/protos/xyz/block/ftl/v1/language;languagepb";
 option java_multiple_files = true;
 
+message Metadata {
+  // Universal metadata
+  string name = 1;
+  string language = 2;
+
+  // Language metadata contains any metadata specific to a specific language.
+  // TODO: revisit type...
+  google.protobuf.Any LanguageMetadata = 3;
+}
+
+message ProjectConfig {
+  string path = 1;
+  string name = 2;
+  bool noGit = 3;
+  bool hermit = 4;
+}
+
+// TODO: docs
+message GetCreateModuleFlagsRequest {
+  string language = 1;
+}
+
+message GetCreateModuleFlagsResponse {
+  message Flag {
+    string name = 1;
+    string help = 2;
+    optional string envar = 3;
+    // short must be a single character
+    optional string short = 4;
+    optional string placeholder = 5;
+    optional string default = 6;
+  }
+  repeated Flag flags = 1;
+}
+
+// Request to create a new module.
+message CreateModuleRequest {
+  string name = 1;
+  string path = 2;
+
+  ProjectConfig projectConfig = 3;
+
+  // TODO: revisit type...
+  google.protobuf.Any Flags = 4;
+}
+
+// Response to a create module request.
+message CreateModuleResponse {}
+
+message DependenciesRequest {
+  string path = 1;
+  Metadata metadata = 2;
+}
+
+message DependenciesResponse {
+  repeated string modules = 1;
+}
+
+// Response to a dependency extraction request.
+message DependencyUpdate {
+  repeated string modules = 1;
+}
+
+// TODO: docs
+message BuildContextUpdatedRequest {
+  // TODO: explain
+  string buildRequestId = 1;
+
+  optional Metadata metadata = 2;
+  optional schema.Schema schema = 3;
+}
+
+message BuildContextUpdatedResponse {}
+
 message Error {
   enum ErrorLevel {
     INFO = 0;
@@ -23,3 +99,103 @@ message Error {
 message ErrorList {
   repeated Error errors = 1;
 }
+
+// Request to build a module.
+message BuildRequest {
+  // TODO: explain
+  string buildRequestId = 1;
+
+  // the root path for the module
+  string path = 2;
+  // the root path for the FTL project
+  string project_path = 3;
+  // indicates whether to watch for changes to the module
+  // TODO: rename to automaticRebuilds
+  bool watch = 4;
+
+  Metadata metadata = 5;
+  schema.Schema schema = 6;
+}
+
+message AutoRebuildStarted {}
+
+// Response to a build request.
+message BuildResult {
+  optional string buildRequestId = 1;
+  // module schema for the built module
+  // only set if the build was successful
+  optional schema.Module module = 2;
+  // paths for files/directories to be deployed
+  repeated string deploy = 3;
+  // errors and warnings encountered during the build
+  ErrorList errors = 4;
+
+  // TODO: dockerfile or docker image name...
+  // TODO: add a way to return what schema / metadata version this was based on
+}
+
+enum LogLevel {
+  DEBUG = 0;
+  INFO = 1;
+  WARN = 2;
+  ERROR = 3;
+}
+
+// Log message from the language service.
+message LogMessage {
+  string message = 1;
+  LogLevel level = 2;
+}
+
+// Every type of message that can be streamed from the language service for a build.
+message BuildEvent {
+  oneof event {
+    DependencyUpdate dependency_update = 1;
+    AutoRebuildStarted auto_rebuild_started = 2;
+    BuildResult build_result = 3;
+    LogMessage log_message = 4;
+  }
+}
+
+// LanguageService allows a plugin to add support for a programming language.
+service LanguageService {
+  // Ping service for readiness.
+  rpc Ping(xyz.block.ftl.v1.PingRequest) returns (xyz.block.ftl.v1.PingResponse) {
+    option idempotency_level = NO_SIDE_EFFECTS;
+  }
+
+  // TODO: docs
+  rpc GetCreateModuleFlags(GetCreateModuleFlagsRequest) returns (GetCreateModuleFlagsResponse);
+
+  // Generates files for a new empty module with the requested name
+  rpc CreateModule(CreateModuleRequest) returns (CreateModuleResponse);
+
+  // Extract dependencies for a module
+  rpc GetDependencies(DependenciesRequest) returns (DependenciesResponse);
+
+  // MetadataUpdated is called whenever the metadata for a module is updated.
+  // TODO: explain this only happens while watching?
+  // TODO: merge with schema updated... to avoid double builds if both change
+  // rpc MetadataUpdated(MetadataUpdatedRequest) returns (MetadataUpdatedResponse);
+
+  // SchemaUpdated is called whenever the relevant part of a schema is updated.
+  //
+  // The schema is not the full schema, rather it only modules in this module's dependency graph.
+  // It does not contain the schema for this plugin's module.
+  //
+  // Language plugins should hold on to the latest schema they have seen.
+  // SchemaUpdated is called:
+  // - after ExtractDependencies and before Build (if there are any dependencies)
+  // - when a dependant module is updated
+  // - when the module's dependancies change
+  //
+  // TODO: explain this only happens while watching?
+  // rpc SchemaUpdated(SchemaUpdatedRequest) returns (SchemaUpdatedResponse);
+
+  rpc BuildContextUpdated(BuildContextUpdatedRequest) returns (BuildContextUpdatedResponse);
+
+  // Build the module
+  // The request can also indicate whether the plugin should watch for changes to the module and automatically rebuild
+  // The response stream can send LogMessages and BuildResults, and ends with a BuildResponse
+  rpc Build(BuildRequest) returns (stream BuildEvent);
+}