Parsing message 3 from longer slice

[chrysn]

When implementing the EDHOC option for CoAP, the user of this library needs to do some CBOR parsing of the message3 structure on their own, just to hand the whole thing over to Lakers, possibly pulling in an own CBOR library.

I don't have a precise plan yet for how to best do this; possibly, a parse_message_3_from_stream() convenience method would do like parse_message_3, but return the consumed length as part of the Ok. (Technically there are some error states in which it could still indicate a parsed length; practically, this is irrelevant because what comes after the EDHOC option can only be used through a successful EDHOC operation anyway).

[geonnave]

I am also not sure on the best way. We could either have a parse_message_3_from_stream() or something like edhoc_with_oscore::parse_comb_payload that directly returns the split msg3 and oscore items.