Credential revocation unable to be expressed as a field constraint

Some credential revocation techniques means that the credential it self does not contain a data element (or field) that is directly revealed when proving whether the credential is revoked or not, instead it can be more associated to the digital signature that is used, e.g as is the case with BBS+ or CL signatures, therefore using a field constraint to express this intent in the request is unsuitable in these cases.

My suggestion is that revocation should be regarded as its own type of constraint rather than as a field constraint, this would manifest as something like the following

```
"constraints": {
"limit_disclosure": true,
"revocation": {
..... information pertaining to the revocation constraint the requestor is placing
},
"fields": []
}
```

The information that needs to be communicated in the revocation block (or else where in the request) is
1. What revocation schemes the requesting party (verifier) supports.
2. What revocation information the verifier is requesting about the credential (e.g does requesting a non-revoked credential mean that credentials that don't include revocation information are included?)