feat: add support for proto3 optional tag

[Dumeng]

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Fixes#726 🦕

The solution is similar to the fix in the Java client SDK.
googleapis/java-bigquerystorage#2295

[Linchin]

Hi @Dumeng, thank you so much for making the contribution! Being compatible with proto3 would be very helpful.

I have a few questions/suggestions:

1. Maybe we can add unit tests and system tests to verify that proto3 is indeed supported
2. Should we consider adding the same treatment for read clients too?

Please let us know if you need any help, we are more than glad to work with you :)

[Dumeng]

Hi @Linchin , Thanks for your reply.

1. The error is coming from the server-side, so the unit test may not be able to verify the fix. I'm not familiar with the system test framework this project is using, I need some research.
2. The ReadRowsStream seems does not need a proto description in its request. The proto description is required and causes the error in the AppendRowsStream.

[Dumeng]

Hi @Linchin , Thanks for your reply.

1. The error is coming from the server-side, so the unit test may not be able to verify the fix. I'm not familiar with the system test framework this project is using, I need some research.
2. The ReadRowsStream seems does not need a proto description in its request. The proto description is required and causes the error in the AppendRowsStream.

[Linchin]

Thanks for adding the system test! I think the system test has failed, and you can see the logs here. We also need to run the linter to pass the lint test.

Also, as you can see we only have tests for v1. Since v1beta1 is going to be deprecated soon, and we don't have tests to verify that it works, I think it's a better idea to restrain our change here to v1.

[Linchin]

Thank you for adding support for proto3!