Add protopluginutil.StripSourceRetentionOptions

[bufdev]

This ports StripSourceRetentionOptions from github.com/bufbuild/protocompile at https://github.com/bufbuild/protocompile/tree/main/options. We had wanted to stick this function somewhere public, and at the time, protocompile was the best option we had. Now, we have a plugin-specific package. This function will also be used in the upcoming protoplugintest package to make it easy to test plugin implementations.

Light edits were made, mostly in service of making the code pass the golangci-lint configuration in this repository.

Sticking this in a new protopluginutil package for now until we figure out the final shape of this repository. I don't think this should live in the main package - we want to make it plugin-authoring specific. Right now, I envision us having:

• protoplugin - author Protobuf plugins
• protoplugintest - test plugins authored with protoplugin
• protopluginutil - useful utilities used by protoplugin and protoplugintest that may have a wider audience. This includes StripSourceRetentionOptions, but also likely ValidateCodeGeneratorRequest, ValidateAndNormalizeCodeGeneratorResponse, and likely a new BuildCodeGeneratorRequest.