Implement the HTTP/2 dataframes encoding/decoding aka compression.

Use the QDecompressHelper to feed the grpc HTTP/2 streams with compressed data. The QDecompressHelper already supports the wellknown compression methods, so we only need to feed it properly according to grpc-accept-encoding/grpc-encoding headers and control the data flow.

Details:
https://github.com/grpc/grpc/blob/master/doc/compression.md