Support uploading structured test gRPC ObjC test results to BigQuery

[dennycd]

Two alternative ways of uploading our test results to BQ

• run_test.py with --bq_result_table (link)
• via bazel post-build script upload_rbe_results.py

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@HannahShiSFB @jtattermusch

[jtattermusch]

For the job being added here: grpc/grpc#29599 the upload_rbe_results.py uploading is already in place (or at least it should be), but the issue is that currently e.g. the entire "UnitTests" target doesn't really display the results in bazel at a test case level (see https://source.cloud.google.com/results/invocations/7432695e-c957-42c3-8869-d8160490a18e/targets/%2F%2Fsrc%2Fobjective-c%2Ftests:UnitTests/tests),
so we might need to experiment with e.g. splitting the UnitTests bazel target into multiple targets (or find some other solution)
https://github.com/grpc/grpc/blob/1d94aa92d883c40abe8b064d79e682f27b432cd3/src/objective-c/tests/BUILD#L181

[jtattermusch]

Also see the comment here: #77 (comment)