Cocoapod build test sample app

[dennycd]

Basic sample app build test for Cocoapod gRPC builds

Change Summary

• Barebone sample app with Podfile for cocoapod builds
  • the sample app takes gRPC & gRPC-ProtoRPC pod dependency which is our V1 & V2 ObjC pods
• build_test_cocoapod_samples build script
• added cocoapod build test to pre submit checks

Test & Verify

• ensure the updated presubmit checks pass on cocoapod sample app build test.

[dennycd]

build test pass at https://github.com/grpc/grpc-ios/actions/runs/3048322281/jobs/4913257403