Support the ability to point to an existing running agent as any role in the tests

[nodlesh]

AS A AATH User, I WANT to pass an endpoint for an existing running agent/backchannel for ACME, Bob, Faber, or Malory, SO THAT I can use agents not managed by AATH and include external agents in the interoperability testing.

This is going to be easier to do with the ACA-Py backchannel than the Credo backchannel. The Credo backchannel essentially is the agent. It calls class methods to tell the agent what to do, not API endpoints like the ACA-Py backchannel does. To do this with Credo we would have to change that implementation or have the backchannel have an api calling mode where it would call endpoints instead of methods if using a remote agent.