[camera]call engine API in main thread to fix a crash

[hellohuanlin]

This PR is part of a larger project to fix the thread model for iOS camera plugin. The formal proposal can be found here

The engine APIs used in camera plugin are required to be run on platform thread. Some of these APIs have explicit asserts (e.g. MethodChannel) and some have warning instructions in documentation (e.g. TextureRegistry). However, in camera plugin we are currently calling these APIs in background thread, causing the crash.

There's already a FLTThreadSafeFlutterResult wrapper to invoke FlutterResult in main thread. This PR created similar "thread safe wrappers" for EventChannel, MethodChannel and TextureRegistry.

Issues

flutter/flutter#94723
flutter/flutter#52578

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[hellohuanlin]

Hi @jmagman @cyanglaz and @stuartmorgan, I re-created this PR from last year.

I left comments about what's updated from the last PR. The biggest change is to use completion block which both @cyanglaz and @jmagman suggested.

PTAL again, thank you!

[stuartmorgan]

LGTM with nits

[hellohuanlin]

FYI I've addressed all nits and will merge in a few hours.