firebase-ios-sdk

Add iOS build support to the CMake build (#5052)

* Use early exit in objc_framework CMakeLists.txt

* Clean up objc_framework CMakeLists.txt

  * Use `version` consistently
  * Use `firebase_ios_version` for the repo-level version
  * Remove INCLUDE directories that are no longer needed
  * Sort dependencies

* Simplify Example/App/CMakeLists.txt

  * Add `OBJC_FLAGS` directly, avoiding a separate `sources` variable.
  * Add resources directly

* Rename test host app to firebase_firestore_example_app

* Add a firebase_ios prefix to functions and variables

This reduces the chance of collision with other projects.

* Simplify compiler_setup

Now that compiler_setup.cmake no longer has side-effects, fold
compiler_id.cmake and archive_options.cmake into it and move
the include of compiler_setup up to the top of the main build.

* Add support for building for iOS

* Make building benchmarks optional

* Disable protoc-based source generators when cross-compiling

* Make downloading googletest optional

* Add DISABLE_STRICT_WARNINGS mode to cc_rules

This makes it possible to build non-Firestore components in the CMake
build to a lower standard than that to which we hold ourselves without
sacrificing the utility of the various build rules that we have.