AuthorizationCredential: need a claim about the scope of access to the resources

[llorllale]

AuthorizationCredential currently has:

1. issuerDIDDoc (which to me means "location" - see AuthorizationCredential: rename "issuerDIDDoc"? #24)
2. requestingPartyDIDDoc (PR fix: term expansion alignment #22)
3. subjectDID (I think should be renamed - see AuthorizationCredential: "subjectDID" #25)

(missing a "resource owner" - see #23)

These things tell us:

1. The location of the resource
2. The requesting party
3. The subject of the claims in the resource

We're missing a way to express the scope of access on the resource along several dimensions:

• time (nbf, exp)
• mode (RO, RW, ...)
• granularity (specific fields inside schemas)
• others?

An initial thought was to use the DIF's presentation exchange format to express this... although it doesn't cover all of the bullets