Propose kotlinx.serialization as 3rd party lib

[nlochschmidt]

Has support for protobuf v2.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

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[nlochschmidt]

I signed it!

[googlebot]

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