Add examples showing an exchange client sending different VPRs with only `acceptedCryptosuites` or `acceptedEnvelopes`

[dlongley]

In the examples, each with VPR should accept any VC that will have one or more matching cryptosuites, per "acceptedCryptosuites", or one matching envelope per "acceptedEnvelopes".

The example should show that a client (e.g., wallet) can start an exchange (if the server supports it) by sending a VPR indicating what they will accept instead of an empty object ({}):

// in this example, the client will accept a number of options from the issuer
{
  "verifiablePresentationRequest": {
    "query": {
      "type": "QueryByExample",
      // allows any VC(s) from the issuer, so no specific "credentialQuery" property;
      // "credentialQuery" ... not present
      "acceptedCryptosuites": [
        "Ed25519Signature2020",
        "eddsa-rdfc-2022",
        "ecdsa-rdfc-2019",
        "bbs-2023",
        "ecdsa-sd-2023"
      ],
      "acceptedEnvelopes": [
        "application/jwt"
      ]
    }
  }
}

// in this example, the client will accept only an enveloped VC using VC-JWT
{
  "verifiablePresentationRequest": {
    "query": {
      "type": "QueryByExample",
      // allows any VC(s) from the issuer, so no specific "credentialQuery" property;
      // "credentialQuery" ... not present
      "acceptedEnvelopes": [
        "application/jwt"
      ]
    }
  }
}

[msporny]

The group discussed this on the 2024-07-23 call:

@dlongley noted that starting an exchange only supports accepting an empty object. We need to support an exchange providing information on what cryptosuites and envelopes it might support (at a minimum), but more generally, we need to support sending an arbitrary VPR so the server can know what to send.

Raise a PR that modifies the OAS to allow an arbitrary VPR to be sent from the client to the server.