Add global search backfill worker (#7860)

* Add search backfill worker

This will run on background at most once an hour while the device has internet connectivity and the device is charging and start back-paginating through the user's rooms to make sure the search results are populated.

It'll run for the maximum time the default workers allow, 10 minutes.

* Create `RustMatrixClient.Factory` AssistedFactory to help with DI and add `SearchBackfillService`

* Start search backfill when entering the search screen, just once. This should help getting initial search results if none or few are found at first

libraries/workmanager/api/src/main/kotlin/io/element/android/libraries/workmanager/api/WorkManagerScheduler.kt

@@ -43,6 +43,7 @@ fun workManagerTag(sessionId: SessionId, requestType: WorkManagerRequestType): S
     val prefix = when (requestType) {
         WorkManagerRequestType.NOTIFICATION_SYNC -> "notifications"
         WorkManagerRequestType.DB_VACUUM -> "db_vacuum"
+        WorkManagerRequestType.SEARCH_BACKFILL -> "search_backfill"
     }
     return "$prefix-$sessionId"
 }
@@ -50,4 +51,5 @@ fun workManagerTag(sessionId: SessionId, requestType: WorkManagerRequestType): S
 enum class WorkManagerRequestType {
     NOTIFICATION_SYNC,
     DB_VACUUM,
+    SEARCH_BACKFILL,
 }