Import room key bundles received after invite. (#5080)
* feat: Import room key bundles when received after invite.
* tests: Add spec test for room key bundle arriving after invite accepted.
* chore: Fix code quality issue (unnecessary async function).
* docs: Tidy up comments.
* refactor: Simplify key bundle importing after invite to one entrypoint.
- Remove `onReceiveToDeviceEvent` from `CryptoBackend`.
- Copy old room key bundle importing logic to
`preprocessToDeviceEvents`.
* refactor: Move late bundle importing to main preprocess loop.
* fix: Use `Map` over `Record` to prevent prototype pollution.
