Handling the frame header

The frame header is made of a frame header descriptor and some other fields, which may or may not be present. The frame header descriptor is a set of flags which indicates which optional fields are present or not.

✅ Implement a frame::FrameHeader structure type containing the fields of a frame header.

✅ Implement FrameHeader::parse() in order to parse the frame header, and write tests to check the parsing behaviour.