Restructure text and add alternative representation

[c2bo]

• fixes a small bug in the PoP examples (backslash)
• Restructures text into 2 major parts
  • header based syntax
  • concatenated representation (<attestation jwt> ~ <attestation jwt pop>)
• adds format & validation similar to an sd-jwt (but simpler)

Rendered version: https://drafts.oauth.net/draft-ietf-oauth-attestation-based-client-auth/c2bo/proposed-format/draft-ietf-oauth-attestation-based-client-auth.html

closes #86
closes #76
probably closes #46?

[paulbastian]

lgtm, should we include that the header based syntax is a MUST implementation support?

[c2bo]

lgtm, should we include that the header based syntax is a MUST implementation support?

I thought about language like that as well, but I don't think it really helps here. It should be in something like HAIP that profiles an ecosystem - a client will/must know what it needs to support and an AS etc. can choose depending on the ecosystem.