Add `<fullExchangeId>/protocols` endpoint that serves interaction "protocols" object

[dlongley]

Performing an HTTP GET on an exchange URL + /protocols should return a "protocols" object with the supported protocols for interacting with that exchange. For example:

GET /workflows/abc/exchanges/123/protocols

{
  "protocols": {
    "vcapi": "https://vcapi.example/workflows/abc/exchanges/123",
    "OID4VCI": "openid-credential-offer://?credential_offer_uri=https%3A%2F%2Fvcapi.example%2Fworkflows%2Fabc%2Fexchanges%2F123%2Fopenid%2Fcredential-offer"
  }
}