Ask for Alternative Sources

Opening this for discussion. Especially if the proposal in #49 is adopted and the Microsub endpoint picks the best feed for the provided input instead of presenting multiple options, there should be a way for the client to tell the endpoint that it wants to be told about alternative sources to the one originally selected, possibly for changing the input.

Start by reading issue #49 and the Microsub endpoint behavior described here. Define how a client would request notification of alternative sources, including whether the input can change; done means the specification has an agreed, precise protocol behavior.

Written by the indexing model from the issue text.