Support DiscardUnknownFieldsParser

[kaelzhang]

Please answer these questions before submitting your issue.

What version of gRPC are you using?

1.14.0

What did you expect to see?

We could disable generating unknownFields from proto messages. It will be great if we could specify the setting by using gradle.

[carl-mastrangelo]

Some thoughts on this:

• Allowing the Marshaller to be modified to discard unknown fields is tricky. The marshaller lives in ProtoLiteUtils but is missing methods for discarding unknown fields. PLU uses ProtobufLite 3.0.~ which is missing the methods
• Allowing a customer CodedInputStream per message might be feasible, perhaps calling into a user provided factory. The issue here is what should be provided to the factory? I was thinking:

interface Factory {
  CodedInputStream(InputStream message);
}

This allows customizing the parsing, but has two drawbacks:

1. The ExtensionRegistry cannot be set this way. It has to be provided to the Parser.
2. A user provided factory cannot take advantage of the buffer caching we do in our parser.

I don't see a clean way of exposing this and allowing code reuse.

[ejona86]

This is very similar to #9643