Implement client creation from an authorization code

[dlongley]

Some protocol message examples below:

  static async fromAuthorizationCode({/*url, agent*/} = {}) {
    /* First get access token from AS:

    POST /token HTTP/1.1
      Host: server.example.com
      Content-Type: application/x-www-form-urlencoded
      grant_type=authorization_code
      &code=SplxlOBeZQQYbYS6WxSbIA
      &code_verifier=dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk
      &redirect_uri=https%3A%2F%2FWallet.example.org%2Fcb
    */

    // FIXME: token response (success); note `c_nonce*` should probably
    // be ignored here as future versions of the OIDC4VCI may address better
    // separating the authorization server and the credential delivery server
    /*
    HTTP/1.1 200 OK
      Content-Type: application/json
      Cache-Control: no-store

      {
        "access_token": "eyJhbGciOiJSUzI1NiIsInR5cCI6Ikp..sHQ",
        "token_type": "bearer",
        "expires_in": 86400,
        "c_nonce": "tZignsnFbp",
        "c_nonce_expires_in": 86400
      }
    */

    // FIXME: token response (failure)
    /*
    HTTP/1.1 400 Bad Request
    Content-Type: application/json
    Cache-Control: no-store
    {
      "error": "invalid_request"
    }
    */

    const client = new OIDC4VCIClient();
    client.accessToken = 'FIXME';
    return client;
  }

[dlongley]

Closing as creation should be done via a credential offer now regardless of grant type.