[camera] Initial iOS Pigeon conversion (#6553)
Converts one platform channel method to Pigeon, setting up all the Pigeon plumbing and test scaffolding.
The Camera API surface is relatively large, so this lays a foundation for incremental conversion, minimizing the mixing of Pigeon setup with the individual method conversions.
Part of https://github.com/flutter/flutter/issues/117905
[camera_avfoundation] Correct pigeon configuration output paths (#9313)
Updates the output paths in the pigeon configuration. I missed this when I separated the implementation into separate targets for SwiftPM (moving the ObjC part into the camera_avfoundation_objc directory).
Resolves: https://github.com/flutter/flutter/issues/168312
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