Support for secret storage (4S)

[poljar]

Secret storage allows us to store the private parts of our cross signing keys and our backup key on the server in the global account data storage box of our user.

This heavily relies on sending requests to the server and getting account data from the sync. Since the crypto crate can't send out requests nor does it have access to the account data it may make sense to just add helpers to the crypto crate and implement the rest of the logic in the top level crate.

Spec entry can be found here: https://spec.matrix.org/unstable/client-server-api/#secret-storage

[jplatte]

The secret storage account data event types have been partially implemented in last year's GSoC, however that work is still stuck in a draft PR because of ruma/ruma#686. were stuck since then and should now (2022-03-24) be implemented soon. Ruma issue: ruma/ruma#478

[tinywombat765]

The linked ruma issue is cloed now. Just wondering what the status of this is?

[poljar]

As mentioned in #1065, no movement as of yet.

[poljar]

Some movement now. PR adding support in the crypto crate: #2591.

[poljar]

This has been now implemented as part of #2621.