fix(database, apple): ensure platform channel is invoked on main thread

[russellwheatley]

Description

Receiving this warning in the console without this fix:

[VERBOSE-2:shell.cc(1004)] The 'plugins.flutter.io/firebase_database' channel sent a message from native to Flutter on a non-platform thread. Platform channel messages must be sent on the platform thread. Failure to do so may result in data loss or crashes, and must be fixed in the plugin or application code creating that channel.
See https://docs.flutter.dev/platform-integration/platform-channels#channels-and-platform-threading for more information.

This just ensures platform channel call in transaction operates on the main thread to comply with Flutter requirement. See documentation about executing platform channel requests on the main thread: https://docs.flutter.dev/platform-integration/platform-channels?tab=type-mappings-obj-c-tab#channels-and-platform-threading

Related Issues

fixes #11628

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