MINOR: sample: Add a protocol buffers specific converter.

MINOR: sample: Add a protocol buffers specific converter.

This patch adds "protobuf" protocol buffers specific converter wich
may used in combination with "ungrpc" as first converter to extract
a protocol buffers field value. It is simply implemented reusing
protobuf_field_lookup() which is the protocol buffers specific parser already
used by "ungrpc" converter which only parse a gRPC header in addition of
parsing protocol buffers message.

Update the documentation for this new "protobuf" converter.