gofeatures: allow setting legacy_unmarshal_json_enum feature at file level

All of the other features, including custom features for C++ and Java,
allow setting a file-wide default. This makes it easier to migrate
proto2 files, so that the feature can be set at the file level instead
of needing to manually annotate each enum element. This adds the same
convenience to the (pb.go).legacy_unmarshal_json_enum feature that is
defined in this repo.

Change-Id: If3a925674a9daeed464b589747b7f78a0200118b
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/589336
Auto-Submit: Lasse Folger <lassefolger@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Lasse Folger <lassefolger@google.com>
Reviewed-by: Michael Stapelberg <stapelberg@google.com>

Author: jhump

cmd/protoc-gen-go/testdata/protoeditions/legacy_enum.pb.go

@@ -0,0 +1,38 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+edition = "2023";
+
+package goproto.protoc.protoeditions;
+
+import "google/protobuf/go_features.proto";
+
+option go_package = "google.golang.org/protobuf/cmd/protoc-gen-go/testdata/protoeditions";
+option features.(pb.go).legacy_unmarshal_json_enum = true;
+
+// EnumTypeWithLegacyUnmarshalJSON comment.
+enum EnumTypeWithLegacyUnmarshalJSON {
+  option features.enum_type = CLOSED;
+
+  // EnumTypeWithLegacyUnmarshalJSON_ONE comment.
+  FIRST = 1;
+  // EnumTypeWithLegacyUnmarshalJSON_TWO comment.
+  SECOND = 2;
+}
+
+message ContainerForNestedEnum {
+  // NestedEnumType1A comment.
+  enum NestedEnum {
+    // NestedEnum_VALUE comment.
+    VALUE = 0;
+  }
+}
+
+enum EnumWithoutUnmarshalJSON {
+  option features.(pb.go).legacy_unmarshal_json_enum = false;
+
+  WITHOUT_UNMARSHAL_JSON_FOO = 0;
+  WITHOUT_UNMARSHAL_JSON_BAR = 1;
+  WITHOUT_UNMARSHAL_JSON_BAZ = 2;
+}

cmd/protoc-gen-go/testdata/protoeditions/legacy_enum.proto

@@ -22,6 +22,7 @@ message GoFeatures {
   optional bool legacy_unmarshal_json_enum = 1 [
     retention = RETENTION_RUNTIME,
     targets = TARGET_TYPE_ENUM,
+    targets = TARGET_TYPE_FILE,
     feature_support = {
       edition_introduced: EDITION_2023,
       edition_deprecated: EDITION_2023,