Add support for custom options

Custom options are a vanilla proto feature documented here. I personally have a use case for them to apply constraints, similar to protoc-gen-validate. I think supporting generic custom options would be a great addition to python-betterproto.

In order for this to happen, I see a couple changes that would be needed:

1. The options proto needs to be compiled by protoc and imported into the namespace from which CodeGeneratorRequest. ParseFromString is called. This req is documented in protocolbuffers/protobuf#3321.
2. plugin.py needs the ability to parse the custom options and somehow store them.
3. We'd need to figure out how to use the custom options: Python has no concept of applying "options" (aside from comments). There are many mechanisms that could be employed (getters/setters, pydantic, comments, etc.). My feeling is that to cover all use cases, plugin.py would need to be made extendable so that individual implementations can decide how to parse custom options and how to use them in the generation of the output .py files.

Is there any interest in this? I'd be willing to try and work on something.

plugin.py と CodeGeneratorRequest.ParseFromString 周辺の options proto の読み込みから始め、次にリンクされている custom-options のドキュメントと protobuf issue を読んでください。生成される Python 出力のために、options をどのように解析、保存、公開すべきかを判断してください。文書化された使用可能な汎用メカニズムが実装され、テストでカバーされた時点で作業は完了です。