tap: gRPC body matching

Allow matching on gRPC message fields.
1) Allow loading descriptor files into the tap filter, similar to how the transcoder works.
2) Using reflection, allow parsing gRPC messages up to a buffer limit (can also work with streaming messages potentially).
3) Via field selection and a TBD matching language, allow matching on individual fields/messages using only loaded descriptor data / reflection.

See https://github.com/envoyproxy/envoy/issues/6108 for JSON matching. It's possible this can be used for that if we consider JSON a Struct.