MINOR: http_fetch: add "req.ungrpc" sample fetch for gRPC.

MINOR: http_fetch: add "req.ungrpc" sample fetch for gRPC.

This patch implements "req.ungrpc" sample fetch method to decode and
parse a gRPC request. It takes only one argument: a protocol buffers
field number to identify the protocol buffers message number to be looked up.
This argument is a sort of path in dotted notation to the terminal field number
to be retrieved.

  ex:
    req.ungrpc(1.2.3.4)

This sample fetch catch the data in raw mode, without interpreting them.
Some protocol buffers specific converters may be used to convert the data
to the correct type.