Implement main App Check classes for iOS

[AlmostMatt]

Description

Implements main App Check logic for iOS

---

Testing

Added integration tests that call SetAppCheckProviderFactory, GetToken and GetTokenLastResult.
Ran app check integration tests locally

---

Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

---

Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[github-actions]

❌  Integration test FAILED

Requested by @AlmostMatt on commit e47a26e
Last updated: Thu Dec 1 13:55 PST 2022
View integration test log & download artifacts

Failures	Configs
firestore	[TEST] [FLAKINESS] [Android] [2/3 os: macos windows] [1/2 android_device: android_target] (1 failed tests)   CRASH/TIMEOUT
gma	[TEST] [FAILURE] [iOS] [macos] [1/2 ios_device: ios_target] (2 failed tests)   FirebaseGmaTest.TestRewardedAdLoad   FirebaseGmaTest.TestRewardedAdLoadEmptyRequest [TEST] [FLAKINESS] [Android] [1/3 os: macos] [1/2 android_device: android_target] (1 failed tests)   FirebaseGmaTest.TestRewardedAdStress

Add flaky tests to go/fpl-cpp-flake-tracker