Cast a coroutine into a Deferred in the federation base

[clokep]

FederationClient.get_pdu was made async/await in #6840 (released in 1.110rc1), but a caller of it was not converted. I'm guessing this was missed because FederationBase._check_sigs_and_hash_and_fetch has an inner function which calls back into FederationBase.get_pdu (see the mypy errors in #6995).

Note that FederationBase._check_sigs_and_hash_and_fetch is also used by FederationServer, so I believe there's potential for another bug here.

Part of #6978.

[clokep]

I was able to reproduce this with SyTest, I think.

[clokep]

An alternative to this might be to convert FederationBase to be async / await, but some of that code is gnarly with the things it does with Deferreds.

[clokep]

We talked a bit about FederationBase calling a FederationClient method on Matrix, wanted to capture the results from rich:

I'm pretty sure I decided that FederationServer.on_query_auth_request had been broken forever and needed to die, which means that _check_sigs_and_hash_and_fetch can move to FederationClient
some more context: _on_query_auth_request implements an undocumented, unused REST endpoint