[JSONSelection] Add ->mapValues method to future:: namespace.

PR #6185 removed support for the following syntax for preserving dynamic
keys of dictionary objects while mapping their values:
```graphql
booksByISBN: books { * { title author }}
```

As suggested in the description of PR #6185, this pattern can be
replaced with a `->mapValues` method call:
```graphql
booksByISBN: books->mapValues(@ { title author })
```

This PR tentatively implements that `->mapValues` method, filing it away
in the `future::` namespace to indicate it hasn't been shipped yet.

apollo-federation/src/sources/connect/json_selection/methods.rs

@@ -55,6 +55,7 @@ lazy_static! {
         public_methods.insert("match");
         // public_methods.insert("matchIf");
         // public_methods.insert("match_if");
+        // public_methods.insert("mapValues");
         // public_methods.insert("add");
         // public_methods.insert("sub");
         // public_methods.insert("mul");
@@ -115,6 +116,11 @@ lazy_static! {
         methods.insert("matchIf".to_string(), future::match_if_method);
         methods.insert("match_if".to_string(), future::match_if_method);
 
+        // Takes an object with unknown keys, binds @ to each of those keys'
+        // values, evaluates the first argument against each @ value, then
+        // produces a new object with the same keys but newly mapped values.
+        methods.insert("mapValues".to_string(), future::map_values_method);
+
         // Arithmetic methods
         methods.insert("add".to_string(), future::add_method);
         methods.insert("sub".to_string(), future::sub_method);

apollo-federation/src/sources/connect/json_selection/methods/future.rs

@@ -3,6 +3,7 @@
 // and tests. After careful review, they may one day move to public.rs.
 
 use serde_json::Number;
+use serde_json_bytes::Map as JSONMap;
 use serde_json_bytes::Value as JSON;
 
 use crate::sources::connect::json_selection::apply_to::ApplyToResultMethods;
@@ -133,6 +134,58 @@ pub(super) fn match_if_method(
     )
 }
 
+pub(super) fn map_values_method(
+    method_name: &WithRange<String>,
+    method_args: Option<&MethodArgs>,
+    data: &JSON,
+    vars: &VarsWithPathsMap,
+    input_path: &InputPath<JSON>,
+    tail: &WithRange<PathList>,
+) -> (Option<JSON>, Vec<ApplyToError>) {
+    if let Some(first_arg) = method_args.and_then(|args| args.args.first()) {
+        if let JSON::Object(map) = data {
+            let mut new_map = JSONMap::new();
+            let mut errors = Vec::new();
+            for (key, value) in map {
+                let new_key = key.clone();
+                let (new_value_opt, value_errors) =
+                    first_arg.apply_to_path(value, vars, input_path);
+                errors.extend(value_errors);
+                if let Some(new_value) = new_value_opt {
+                    new_map.insert(new_key, new_value);
+                }
+            }
+            tail.apply_to_path(&JSON::Object(new_map), vars, input_path)
+                .prepend_errors(errors)
+        } else {
+            (
+                None,
+                vec![ApplyToError::new(
+                    format!(
+                        "Method ->{} requires an object input, not {}",
+                        method_name.as_ref(),
+                        json_type_name(data),
+                    ),
+                    input_path.to_vec(),
+                    method_name.range(),
+                )],
+            )
+        }
+    } else {
+        (
+            None,
+            vec![ApplyToError::new(
+                format!(
+                    "Method ->{} requires exactly one argument",
+                    method_name.as_ref()
+                ),
+                input_path.to_vec(),
+                method_name.range(),
+            )],
+        )
+    }
+}
+
 pub(super) fn arithmetic_method(
     method_name: &WithRange<String>,
     method_args: Option<&MethodArgs>,

apollo-federation/src/sources/connect/json_selection/methods/tests.rs

@@ -362,6 +362,67 @@ fn test_match_methods() {
     );
 }
 
+#[test]
+fn test_map_values_method() {
+    assert_eq!(
+        selection!("$->mapValues(@->add(10))").apply_to(&json!({
+            "a": 1,
+            "b": 2,
+            "c": 3,
+        })),
+        (
+            Some(json!({
+                "a": 11,
+                "b": 12,
+                "c": 13,
+            })),
+            vec![],
+        ),
+    );
+
+    assert_eq!(
+        selection!("$->mapValues(@->typeof)").apply_to(&json!({
+            "a": 1,
+            "b": "two",
+            "c": false,
+        })),
+        (
+            Some(json!({
+                "a": "number",
+                "b": "string",
+                "c": "boolean",
+            })),
+            vec![],
+        ),
+    );
+
+    assert_eq!(
+        selection!("$->mapValues(@->typeof)").apply_to(&json!({})),
+        (Some(json!({})), vec![]),
+    );
+
+    assert_eq!(
+        selection!("$->map(@->mapValues(@->typeof))").apply_to(&json!([])),
+        (Some(json!([])), vec![]),
+    );
+
+    assert_eq!(
+        selection!("$->map(@->mapValues(@->typeof))").apply_to(&json!([{
+            "x": 1,
+            "y": "two",
+            "z": false,
+        }])),
+        (
+            Some(json!([{
+                "x": "number",
+                "y": "string",
+                "z": "boolean",
+            }])),
+            vec![],
+        ),
+    );
+}
+
 #[test]
 fn test_arithmetic_methods() {
     assert_eq!(