[Backend Tester] Add CoreML tester implementation

[GregoryComer]

Add a tester implementation for Core ML. This uses the common backend tester framework to allow Core ML to plug into the test harness using pybindings.

[GregoryComer]

Stack from ghstack (oldest at bottom):

• [Backend Tester] Add compliance suite skeleton and operator tests #11960
• -> [Backend Tester] Add CoreML tester implementation #11959
• [Backend Tester] Add FACTO operator test skeleton #11953

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/11959

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

✅ You can merge normally! (1 Unrelated Failure)

As of commit a931676 with merge base 91c9ffa ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-mediatek-models-linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

This comment was automatically generated by Dr. CI and updates every 15 minutes.