Add a search backfill app background processing task

To run when the device is idle, charging and connected to the network.

ElementX/SupportingFiles/Info.plist

@@ -5,6 +5,7 @@
 	<key>BGTaskSchedulerPermittedIdentifiers</key>
 	<array>
 		<string>io.element.elementx.background.refresh</string>
+		<string>io.element.elementx.background.search-backfill</string>
 	</array>
 	<key>CFBundleDevelopmentRegion</key>
 	<string>$(DEVELOPMENT_LANGUAGE)</string>

ElementX/SupportingFiles/target.yml

@@ -100,7 +100,8 @@ targets:
           location
         ]
         BGTaskSchedulerPermittedIdentifiers: [
-          io.element.elementx.background.refresh
+          io.element.elementx.background.refresh,
+          io.element.elementx.background.search-backfill
         ]
         UTExportedTypeDeclarations:
         - UTTypeConformsTo: [public.text]