feat(genaip): add support for Protobuf Editions

[vallahaye]

Hello again 👋

These changes are enough to add support for Protobuf Editions (and proto3 optional fields) in protoc-gen-aip-go.

Let me know if this works for you and if I should add simple tests (although it doesn't affect the codegen).

Closes #327

[alethenorio]

Hi

Thank you for the PR. Can you detail the use case for introducing this change?

While I agree that it would be nice to have support for editions for a more future-safe setup, the AIP syntax spec explicitly states that APIs should be written in proto3 format so maybe we should start there and get a better understanding on whether there are any plans supporting editions?

[vallahaye]

Hello @alethenorio

Thank you for the quick reply. To be totally honest, I overlooked that part of the spec and, after reviewing it, you're absolutely right:

APIs defined in protocol buffers must use proto3 syntax.

So, at the bare minimum, the changes I committed should not enable proto2 in protoc-gen-aip-go. Regarding Editions, I did a little research on the AIPs' GitHub repository and couldn't find anything mentioning upcoming support or the opposite. No issue, no PR, no discussion, nothing.

In summary:

• proto2 should not be supported (I'll bump the SupportedEditionsMinimum property in this regard).
• proto3 optional fields, although not explicitly mentioned, should be supported in my opinion. I found one Google Cloud specific AIP making use of the keyword.
• It's up to you to decide if you want to deviate a bit from the specification and support Editions now for the sake of future compatibility. If it were me, I'd do it, as I can definitely see new projects embracing the Connect RPCs stack, Protobuf Editions (which is now mature) to later discover AIPs and get stuck with protoc-gen-aip-go not being able to handle their Protobuf files.