encoding: Add support for identity encoding.

[tsuna]

This fixes #1654

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[tsuna]

I signed the Linux Foundation CLA. Also working to get a Corporate CLA in place between Arista and CNCF.

[dfawley]

Thanks for the contribution! My fix for this is #1664. There were a number of minor problems and inconsistencies with the existing code in grpc for applying the compressor/decompressor; I believe I've ensured everything works correctly and consistently now.