Include optional `credentialOffer` property in a VPR that announces VCs to be issued in step

[dlongley]

When a VPR is provided to a recipient, it could include a credentialOffer property that mentions potential VCs that might be issued if the VPR is responded to. This can also give a responder the opportunity to provide their own VPR with constraints on what they'd like to receive, if for example, there are several options on offer, and/or give them the opportunity to provide necessary information to receive what they are interested in.

This could be used to signal to a client that a VC that uses BBS is available, but that the client needs to provide some BBS-specific commitments (see #396).

An example of using this property might look like this:

{
  "query": [/* queries */],
  "domain": "...",
  "challenge": "...",
  "credentialOffer": [{
    "mediaType": "application/vc",
    "credential": {
      // minimally contextualized-type information, but other information
      // could also be present
      "@context": [
        "https://www.w3.org/ns/credentials/v2",
        "https://www.w3.org/ns/credentials/examples/v2"
      ],
      "type": "ExampleAlumniCredential",
      "proof": [{
        "type": "DataIntegrityProof",
        "cryptosuite": "bbs-2023"
      }, {
        /* other proofs on offer */
      }]
    }
  }, {
      /* other offered credentials */ 
  }]
}

[dlongley]

We might eventually want to specify (or just allow incubation of this to happen) other fields inside credentialOffer values such as requiredGroup (or a better name) that refer to groups in the VPR that must be satisfied in order to obtain the credential.