Refactor AgentInterfaceFactory to use a Plugin Architecture

[nodlesh]

AS A AMTH user with a private test repo and wallet app, I WANT to be able to use my own Issuer and/or Verifier interfaces with AMTH without having to modify the the public agent factory in AMTH, SO THAT our internal details of our agents remain private.

ACCEPTANCE CRITERIA
Given a AMTH User with Private Issuer and Verifier Agents
And they have implemented AMTH agent interfaces for these Agents
| issuer_agent | verifier_agent |
| MyPrivateIssuerAgentInterface | MyPrivateVerifierAgentInterface |
When I run a script to modify the agents json file to add my agent interfaces
And I copy my agent interfaces into the AMTH structure
And I run AMTH with the -i "MyPrivateIssuer;<myprivateissuer endpoint>" -v "MyPrivateVerifier;<myprivateverifier endpoint>"
Then AMTH will run my wallet tests with my Agent interfaces

One possible way to accomplish this is to use a plugin architecture in Python utilizing importLib.