MINOR: sample: Rework gRPC converter code.

MINOR: sample: Rework gRPC converter code.

For now on, "ungrpc" may take a second optional argument to provide
the protocol buffers types used to encode the field value to be extracted.
When absent the field value is extracted as a binary sample which may then
followed by others converters like "hex" which takes binary as input sample.
When this second argument is a type which does not match the one found by "ungrpc",
this field is considered as not found even if present.

With this patch we also remove the useless "varint" and "svarint" converters.

Update the documentation about "ungrpc" converters.