[camera] Finish converting iOS to Pigeon (#6601)
Converts all remaining Dart->host communication in the iOS implementation to use Pigeon. Given the boilerplate nature of many of the changes, it seemed easiest to just do the remaining calls all at once now that the structure is in place.
Some high-level notes:
- Many methods used to send the cameraId without it ever being used on the native side, so the Pigeon versions do not send them.
- ThreadSafeTextureRegistry is removed because I discovered that it was masking a bug, so was more trouble than it was worth (see inline comments in PR).
- A number of enums have been removed in favor of using the Pigeon-generated enums to pass data from the plugin class to FLTCam.
- In many cases where the completion callback (previously result) was being passed to FLTCam in a call, only to have it always just call result(nil), that's now done in the plugin class since it's easier to reason about completions being called when they aren't passed around. (Long term we should consider moving almost all of the rest out, and using FlutterError* out params that the plugin class passes to completion, but that is more surgery than I wanted to do in this PR.)
Completes the iOS portion of https://github.com/flutter/flutter/issues/117905