MINOR: sample: Replace "req.ungrpc" smp fetch by a "ungrpc" converter.

MINOR: sample: Replace "req.ungrpc" smp fetch by a "ungrpc" converter.

This patch simply extracts the code of smp_fetch_req_ungrpc() for "req.ungrpc"
from http_fetch.c to move it to sample.c with very few modifications.
Furthermore smp_fetch_body_buf() used to fetch the body contents is no more needed.

Update the documentation for gRPC.