Bazel build test migration for Sample Apps

[dennycd]

Migrate cocoadpod-based sample app build test to bazel based. Including the following project

• Objc Sample App
• Swift Sample App

Kokoro integration components at

• https://github.com/grpc/grpc/blob/1cd6e69347cbf62a012477fe184ee6fa8f25d32c/tools/run_tests/run_tests.py#L996-L1016

We can utilize existing bazel bulid ruled defined at

• https://github.com/grpc/grpc/blob/master/src/objective-c/examples/BUILD

And the corresponding bazel build test script at

• https://github.com/grpc/grpc/blob/master/src/objective-c/tests/build_one_example_bazel.sh

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cc @jtattermusch

[jtattermusch]

//src/objective-c/examples:Sample and //src/objective-c/examples:tvOS-sample bazel targets already exist and are being run here: grpc/grpc#29599.

For the remaining samples we either don't have a bazel target or the bazel target is broken.

[jtattermusch]

I'm adding TODOs for the remaining tasks here: https://github.com/grpc/grpc/pull/29665/files#diff-26e0bc5bba55be18a1e28b707c1768be5c3de415944115ec3b4eb006b1d39693