Element-R: Automatically import keys from backup after gossip (following interactive verification)

[BillCarsonFr]

Fixes https://github.com/vector-im/element-web/issues/26312

Introduces a new CryptoEvent that is fired when the backup decryption keys is received. When received by the matrix client, an full backup import is started.

The event is only fired by the rust crypto backend to avoid modifying legacy backend.

Checklist

• [ ] Tests written for new code (and old code if feasible)
• [ ] Linter and other CI checks pass
• [ ] Sign-off given on the changes (see CONTRIBUTING.md)

---

This change is marked as an internal change (Task), so will not be included in the changelog.