Element R: set up cross signing during registration

[richvdh]

• Add checks to the cypress test for registration (register.spec.ts) - download the cross-signing keys for the user, and the device keys, and check that the device appears to be cross-signed. (Don’t bother with a cryptographic check: it is likely valid if signed at all) (Cypress test to check cross-signing keys on register matrix-org/matrix-react-sdk#10850)
• Define MatrixClient.bootstrapCrossSigning in CryptoApi, with an implementation in RustCrypto. (Element-R: initial implementation of bootstrapCrossSigning matrix-org/matrix-js-sdk#3368)
  • Call OlmMachine::crossSigningStatus to check that the OlmMachine does not yet have X-Signing keys
  • Pull out CrossSigning.isStoredInSecretStorage to somewhere common. Use it to check that the keys are not stored in secret storage.
  • Call olmMachine.bootstrapCrossSigning.
  • Process the OutgoingRequests. Note that they may require UIA
• js-sdk integration test, to run on both legacy and rust crypto (Integration test for bootstrapCrossSigning matrix-org/matrix-js-sdk#3355)
  • Call bootstrapCrossSigning, and make sure that cross-signing keys get uploaded, and a device signature.