Allow exe/protoboeuf to function as a protoc plugin

[tenderworks]

exe/protoboeuf currently works as a stand-alone executable that can read binary protobuf ASTs. I think it would be useful if it was also able to function as a protoc plugin. This way people can integrate our codegen with existing protoc workflows.

[colinbendell]

You will need this if you want to get access to any source-only option fields (For example: https://github.com/Shopify/proto/blob/main/shopify/session/annotations.proto#L13-L16 )

extend google.protobuf.FieldOptions {
  // docs for a session field
  SessionDocs docs = 99901 [retention = RETENTION_SOURCE];
}

These extensions are available in the CodeGeneratorRequest message along with comments. Of course this does mean that you will need to generate and eval both the request and all options at the time of codegen. You can see how we do this in the codegen process for nuevo where we have to load the dependent objects, compile them and then read the options from the descriptor pool