support decoding gRPC messages with tap filter

By being given access to the proto descriptor the HTTP tap filter should be able to output gRPC message bodies as human readable (i.e. as JSON).

Note the overlap with https://github.com/envoyproxy/envoy/issues/6109: both issues would rely on the proto descriptors being made available to Envoy (either statically or dynamically somehow), but this issue tracks being able to support a gRPC specific output format rather than the ability to match on message fields.