Add recovery key handling to native crypto to keep element compatibility and support MSC4153

[MTRNord]

MSC4153 seems to actively break crypto of bots as element's labs implementation has started to block non cross-signed devices from being readable. Tulir pointed out that recovery keys are the way to go here.

See also matrix-org/matrix-spec-proposals#4153 (comment)

This might need a rather timeish fix or workaround to not break people's bots since (having used matrix long enough) element likely will push this down their stable clients way before the MSC is stabilized. :/

[MTRNord]

Currently having this MSC in the labs enabled makes this with a pantalaimon setup:

It is very likely also causing this on native crypto since we dont take any other steps either.

[Gnuxie]

💀