feat: first cut at object detection on ios

[cdiddy77]

Object detection inference working (a little) on ios

Similar to PR #11 this is exactly enough code to prove that we are getting inferences from mediapipe on iOS

What is working

• If you run the demo app, you may see a very slow frame rate camera. More significantly, in the logging terminal, you will see logging spew indicating that the javascript layer is receiving inference results from the ios layer. These inferences may or may not be correct. But they are inferences that are the result of sending video frames to mediapipe's ObjectDetection task.

What needs doing

• Need to verify that the camera frame rate is still 30fps, even though the inferences aren't flowing quite that fast.

The details

• On iOS, similar to Android, there is an ObjectDetectionModule and an ObjectDetectionFrameProcessorPlugin. ObjectDetectionModule has methods for creating and releasing an ObjectDetector, and fires events when object detectors produce results or errors. The frame processor plugin forwards the frame to the underlying ObjectDetectorHelper which forwards to MediaPipe. When MediaPipe generates a result or an error, those are forwarded through the event mechanism.