Support the gossiping of cross signing keys.

[poljar]

The spec has been expanded so different secrets besides a room key can be transferred between devices.
The relevant spec entry can be found here.

We should add support to request and send private parts of the cross signing keys, for this the KeyRequestMachine should be generalized so it supports all the different secrets.

Related Ruma PR: ruma/ruma#677

[jplatte]

ruma/ruma#677 has been merged and released in ruma-events 0.24.3 🙂

[poljar]

Oh awesome, thanks a bunch for the quick release.