test(proto): generate tests for AppendRequest and ReplicateRequest

This PR adds generated tests for
`github.com/kakao/varlog/proto/snpb.AppendRequest` and
`github.com/kakao/varlog/proto/snpb.ReplicateRequest`. These tests are generated
using the [gogoproto testgen tag](https://pkg.go.dev/github.com/gogo/protobuf/plugin/testgen).

Resolving issue #795 may require changes to the unmarshaling code generated by
gogoproto. Adding these tests will provide confidence when making those changes.

proto/snpb/log_io.proto

@@ -20,6 +20,10 @@ option (gogoproto.goproto_sizecache_all) = false;
 // AppendRequest is a message to send a payload to a storage node. It contains
 // a vector of storage nodes to replicate the payload.
 message AppendRequest {
+  option (gogoproto.equal) = true;
+  option (gogoproto.populate) = true;
+  option (gogoproto.testgen) = true;
+
   int32 topic_id = 1 [
     (gogoproto.casttype) = "github.com/kakao/varlog/pkg/types.TopicID",
     (gogoproto.customname) = "TopicID"