Add proto3_json option for conformant JSON enum serialization

[smparkes]

[This is similar to #589 but that's pretty old. Tried to make this very straightfoward.]

The proto3 JSON spec requires enum values to use their original name from the .proto file (e.g. "SUIT_HEARTS"). betterproto stripped the enum type prefix for Python ergonomics (PR #187), which also changed JSON output to use stripped names ("HEARTS") — breaking interop with other proto3 JSON implementations like protobuf-es.

Add a proto3_json parameter to to_dict() that, when True, uses the original .proto enum names. The default (False) preserves existing behavior for backwards compatibility.

Changes:

• Add proto_name attribute to Enum members (original .proto name)
• Add proto_names class dict populated by codegen template
• Add proto3_json parameter to to_dict() (default False)
• Update from_string() to accept both Python and proto names
• Update from_dict() to accept integer enum values
• Update codegen template to emit proto_names mapping
• Update codegen model to carry proto_name through EnumEntry
• Update README JSON section with proto3_json documentation
• Add test_proto3_json.py with serialization, deserialization, round-trip, and backwards compatibility tests

Summary

Add support for strict proto3 JSON

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
  • This change has an associated test.
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)