Setup ios bazel test to run c-core's existing event_engine unit test

Setup bazel [ios_unit_test](https://github.com/bazelbuild/rules_apple/blob/master/doc/rules-ios.md#ios_unit_test) and test against C-core's existing event engine test suite

Existing c-core unit test on iOS is done via the following bazel rule
* [ios_cc_test](https://github.com/grpc/grpc/blob/master/bazel/grpc_build_system.bzl#L195)
- this is added for [each grpc_cc_test target](https://github.com/grpc/grpc/blob/master/bazel/grpc_build_system.bzl#L278)

on iOS/Mac, we run the following script to trigger bazel test on kokoro CI

* [grpc_run_bazel_cpp_ios_test.sh](https://github.com/grpc/grpc/blob/master/tools/internal_ci/macos/grpc_run_bazel_cpp_ios_tests.sh)

This is test against all ios_unit_test targets under //test/cpp. We can setup a similar structure for event engine test target
* https://github.com/grpc/grpc/blob/master/test/core/event_engine/BUILD#L21

----

Sample instruction to run bazel ios test in command line

```bash
tools/bazel query "kind(ios_unit_test, tests(//test/core/event_engine/...))"

tools/bazel test --test_summary=detailed --test_output=all //test/core/event_engine:endpoint_config_test_on_ios

```