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.
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