Expose extended options to plugins

[kerinin]

It's possible there's a way to do this that I've just missed, but...

Given a custom option like so:

syntax = "proto3";

import "google/protobuf/descriptor.proto";

extend google.protobuf.FileOptions {
  string foo = 50001;
}

option (foo) = "bar";

It would be really nice if the value bar could be extracted from an instance of plugin.CodeGeneratorRequest. The value exists in plugin.CodeGeneratorRequest.ProtoFile.*.Options.XXX_InternalExtensions, however there's no way to access it - proto.GetExtensions requires a proto.ExtensionDesc, but the values returned by proto.ExtensionDescs and friends return values containing just the "Field" field, which causes a panic if passed to proto.GetExtensions. In theory the extensions could be registered by importing the generated code, but that can't be done dynamically.

WRT motivation for such a change, I'm working on a documentation generator and I would like to do things like:

• Annotate fields as "private" to omit them from generated docs
• Annotate fields for generating "fake" example data, ie string address = 1 [(generator) = "email"];
• Annotate the authentication requirements of RPC services and methods
• Annotate endpoints for RPC services
• Annotate field / message validation constraints

[dsnet]

Support for plugins in general needs to be discussed. If we decide to provide first-class support for plugins, there will probably be an entire redesign.

Closing this in favor of the umbrella issue #547.