firebase-ios-sdk

Shorten CMake Objective-C targets (#5323)

* Shorten firebase_firestore_example_app

... and:
  * Migrate to firebase_ios_add_executable
  * Use globs for specifying sources

* Migrate Example/Tests/API to new CMake primitives

* Migrate Example/Tests/Integration to new CMake primitives

* Migrate Example/Tests/SpecTests to new CMake primitives

* Migrate Example/Tests/Util to new CMake primitives

Rename firebase_firestore_objc_test_util to firestore_objc_testing.

* Migrate fuzzing to new CMake primitives

Rewrite firebase_ios_cc_fuzz_test to match the other primitives.

* Add firebase_ios_add_objc_test

* Migrate Example/Benchmarks to new CMake primitives