E2E Encryption: Support more keyring strategies

We have a platform which runs a WAMP router, that system also has a secure element. each user (mobile phone / website) has their own public/private key generated on login. The current implementation in autobahn ties the Key (mix of originator and responder keys) to a URI.

This is mostly fine however in our case it is desired to have 1 responder and multiple originators, the originators can dynamically change as people are added/removed from the system.

From the look of it, it is currently not possible with the autobahn-python API. There are two changes that I think will make the above possible

1. The Key object should support multiple originators (a list)
2. the enc_key in the Invocation should be used to determine if the originator is "expected"

Start by tracing the Python implementation of the Key object and Invocation enc_key handling, then review related tests and API usage. Determine how key associations currently work and whether the proposed originators list and expected-originator check fit the existing design. Done means supporting one responder with dynamically changing originators without breaking current key behavior.