Replaces []Format with a Group type.
A bit more type safe.
Breaking change for RegisterFormat, now takes a first argument that is a "single" format group.
Lots of naming cleanup.
This is also preparation for decode group argument which will enable doing intresting
probing, ex a format decoder could know it's decode as part of probe group (html could
be probed possibly), or have "arg probe" group for decoder who inspect args to know
if they should probe (-d /path/to/schema etc) to enable nice CLI-ergonomics.
This will allow passing both cli options and format options to sub decoder.
Ex: pass keylog option to a tls decoder when decoding a pcap.
Ex: pass decode options to a format inside a http body inside a pcap.
Add ArgAs method to lookup argument based on type. This also makes the format
decode function have same signature as sub decoders in the decode API.
This change decode.Format a bit:
DecodeFn is now just func(d *D) any
DecodeInArg renamed to DefaultInArg
