Could reduce restatement of COSE requirements

[laurencelundblade]

There's a number of places where COSE requirements are unnecessary restated, referenced, repeated. I think they can be removed and the document would be shorter and more clear. COSE-HPKE doesn't change these characteristics of COSE. When they are restated, there's the risk of restating them wrong or leading the reader to think there's something different when there isn't.

• All discussion of CBOR tags can be removed
• All discussion of detached vs inline payload can be removed
• The CDDL for a COSE_SIgn1 can be removed (sufficient to say that HPKE-base provides no auth)
• There's some restatement on how Enc_structure works. It is exactly the same as COSE, right? It really needs to be.
• There's restatement of how external_aad works