…d code (apple#3089)

### Motivation

The generated ServiceTalk gRPC stubs make use of deprecated calls. This causes a problem when `-Werror` is used to build the code since it will automatically fail the build. We should allow users who have already migrated their code to prevent the protoc compiler from generating and using deprecated references.

### Modifications

Add an option, `skipDeprecated`, as part of the protoc compiler configuration, to tell the generator to leave out deprecated references.

Remove
- initSerializationProvider, reason: ContentCodec is deprecated
- isSupportedMessageCodingsEmpty, reason ContentCodec is deprecated
- Deprecated ServiceFactory constructors
- ServiceFactory::Builder references to ContentCodec
- Generated client metadata and associated methods, reason: deprecated

### Result

Users can generate gRPC stubs without deprecated code.

### Testing

Manually tested:
- Add new option to ServiceTalk/examples/grpc/protoc-options and use it to generate test_service.proto which covers all combinations of streaming (or not) services.

This reverts commit 2c4b42c.

…entation"

This reverts commit 49ebbe2.