crypto: Add new `ClientEvent.ReceivedToDeviceMessage` with proper `OlmEncryptionInfo` support (#4891)
* crypto: Add new ClientEvent.ReceivedToDeviceMessage
refactor rename ProcessedToDeviceEvent to ReceivedToDeviceEvent
* fix: Restore legacy isEncrypted() for to-device messages
* Update test for new preprocessToDeviceMessages API
* quick fix on doc
* quick update docs and renaming
* review: Better doc and names for OlmEncryptionInfo
* review: Remove IToDeviceMessage alias and only keep IToDeviceEvent
* review: improve comments of processToDeviceMessages
* review: pass up encrypted event when no crypto callbacks
* review: use single payload for ReceivedToDeviceMessage
* fix linter
* review: minor comment update