[Test] Introduce SPM test sample app

[dennycd]

Manually created XCode sample test app project for swift package manager build test.

• Manual xcode project reflects default app/xcode build settings as used by gRPC/iOS clients.
• The ios app locally depends on grpc-ios/Package.swift and allows us to easily test local dev modification and in CI.
• xcconfig build setting to customize and turn on the following flags
  • GCC_TREAT_WARNINGS_AS_ERRORS=YES to treat warning as erros

Also introduce build_test_spm_samples.sh for command line build test via xcodebuild

Build and Verify

run build test script and verify success

./scripts/build_test_spm_samples.sh

[HannahShiSFB]

May I ask why set GCC_TREAT_WARNINGS_AS_ERRORS = YES in both scripts/build_test_spm_samples.sh and tests/spm/gRPCSample/build.xcconfig?

[dennycd]

May I ask why set GCC_TREAT_WARNINGS_AS_ERRORS = YES in both scripts/build_test_spm_samples.sh and tests/spm/gRPCSample/build.xcconfig?

xcconfig can only be applied to project target (all sample app targets) but not to its dependencies (e.g. gRPC-Cpp). For that we need to pass the preprocessor macro via xcodebuild comamnd line and populate it upstream.