Create Aries RFC for new AnonCreds credential and proof format based on RFC 0592

[TimoGlastra]

• Can be a clone of indy RFC, but minor changes based on spec (so id values that are allowed)
• Add pointers to the spec instead of indy-sdk rust code

RFC 0592: https://github.com/hyperledger/aries-rfcs/blob/main/features/0592-indy-attachments/README.md

DoD: PR open