Expand nonce proteceted header definition; define CBOR data format

[dwaite]

Describe the CBOR protected header as containing a binary string.

While in the section, ellaborate usage of the nonce protected header.

[dwaite]

I ultimately decided to stick with the simpler form until I hear use cases otherwise - in particular, the EATS document defining multiple nonce values in the CBOR claim:

1. Does not give any justification for wanting such flexibility
2. Does not provide examples of such flexibility in use