Add `decode_format` to allow incoming messages to be decoded into maps

[surik]

Diameter 2.1 has this option. The change log for OTP 20.1 says:

Add service option decode_format to allow incoming messages to be decoded into maps
instead of records.

Would be nice to have something like this in eradius.

[RoadRunnr]

nice to have, but IMHO not urgent