remove gRPC size limit for bigquery storage

[MartinNowak]

• currently limited to 4MiB
• compare to f636c8d which does the same for the monitoring API

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[tswast]

@crwilcox Could you review this? Seems okay once @MartinNowak signs the CLA, but I don't know if synthtool is just going to overwrite it.

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[tswast]

Thanks for the contribution @MartinNowak. FYI: I've added this to synth.py as well so that the code generator doesn't override your changes.