Enable direct access to utility methods in ProtobufData

Enable direct access to utility methods in ProtobufData that deal with the message description on a more granular level.

ProtobufData presents some great functionality that allows dealing with the Protobuf message descriptors. The intention of this PR is to provide access to the methods that deal with these descriptors on a more granular level. This would make it more flexible for the library clients to deal with their Protobuf message.

In terms of extending the functionality this would make it possible for clients to adjust some specific moments of the conversion from Protobuf descriptors to Avro schemas.