New ignore_comments option for excluding proto comments from OpenAPI output

[keyz]

References to other Issues or PRs

Fixes #2684

Have you read the Contributing Guidelines?

Yes

Brief description of what is fixed or changed

It's pretty common to have proto comments that are irrelevant to OpenAPI output. For instance, if you have a // buf:lint:ignore comment, the only way to exclude that today is to manually add an override via grpc.gateway.protoc_gen_openapiv2.options.openapiv2_field per #2684 (comment).

In this PR I'm adding a new config option: ignore_comments. It's disabled by default and only affects the OpenAPI output; when you enable the option, all proto comments are excluded from the output. With the new option on, annotations can still be explicitly added via grpc.gateway.protoc_gen_openapiv2.options.openapiv2_field.

Let me know if you have any feedback!

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[johanbrandhorst]

I can actually get behind this option. It's consistent and reasonably isolated from other functions. I would request three things:

1. Lets just call it ignore_comments.
2. Please add a check to startup that errors if both ignore_comments and use_go_template is set. These are mutually exclusive options.
3. Lets add something to the docs, specifically we usually have a section for each flag in https://github.com/grpc-ecosystem/grpc-gateway/blob/main/docs/docs/mapping/customizing_openapi_output.md.
   Thank you!

[keyz]

@johanbrandhorst sounds good -- just updated the PR. Thank you!

[johanbrandhorst]

LGTM, thank you!