Support recently added "source_file_descriptors" field of CodeGeneratorRequest

[jhump]

I've also made sure this branch can apply to bufmod. Here's my tentative cherry-pick:
bufmod...jh/strip-source-only-on-bufmod

[bufdev]

An overall thing: I think we might want to make it so that we can select source-retention options or not at the ImageFile level. A proposal for how I'd do this:

type ImageFile interface {
  ...
  FileDescriptorProto() *descriptorpb.FileDescriptorProto
  FileDescriptorProtoWithoutSourceRetentionOptions() (*descriptorpb.FileDescriptorProto, error)
  ...
}

Then, in imageFile, I would do:

type imageFile struct {
  ...
  getFileDescriptorProtoWithoutSourceRetentionOptions func() (*descriptorpb.FileDescriptorProto, error)
}

And on construction:

func newImageFileNoValidate(...) {
  imageFile := &imageFile{...}
  // Make sure to use syncext instead of sync as OnceValues was not in Golang <1.21.
  imageFile.getFileDescriptorProtoWithoutSourceRetentionOptions =syncext.OnceValues(imageFile.getFileDescriptorProtoWithoutSourceRetentionOptionsUncached)
  return imageFile
}

And at the bottom of image_file.go, before isImageFileInfo/isImageFile:

func (f *imageFile) getFileDescriptorProtoWithoutSourceRetentionOptionsUncached() (*descriptorpb.FileDescriptorProto, error) {
  return stripSourceRetentionOptions(f.fileDescriptorProto)
}

[jhump]

An overall thing: I think we might want to make it so that we can select source-retention options or not at the ImageFile level.

@bufdev, this approach could expose the functionality for a *descriptorpb.FileDescriptorProto. But we'll also need this functionality for *imagev1.ImageFile (so we can optionally exclude these options from the output of buf build). So, in ca03cd0, I instead opted to export StripSourceRetentionOptionsFromFile and make it generic so that it can handle either representation of a file. WDYT?

Also, I tackled the TODO for more validation of the CodeGeneratorRequest in 6e767e4. WDYT? Too much? I can easily back it out if so.

[bufdev]

Thanks for getting through this! Merge into bufmod after we merge here.