Mock out gateway <-> federation network layer

[justinmoon]

We should be able to test the interaction use just a rust channel or something which is independent any networking details. This might make tests run a little faster, too!

Notes:

• https://github.com/fedimint/fedimint/pull/427#issuecomment-1225381751
• @elsirion any other notes about this?

[elsirion]

Feels related to the module interconnect too, might be worth taking a look at that when building the API mock. https://github.com/fedimint/fedimint/blob/3c60fb65e58e11f2f7bf94c2920f9acace0e9d8c/fedimint-api/src/module/interconnect.rs#L5-L16

[okjodom]

Currently looking at this as part of #742