Support OpenAPI schema annotations on enum types

[jalaziz]

OpenAPI v3 annotations currently support files, methods, messages, and message fields, but not protobuf enum types.

Because protoc-gen-openapi generates enum schemas inline, there is no simple way to attach enum-level schema metadata (e.g. title, description, example, or default). In our case, this prevents downstream documentation renderers from using those values when generating examples for enum-typed fields.