Update google-cloud-rust to use Protobuf 3.0

[data-sync-user]

The GCP implementations we use for Bigtable and Sync storage are using older protobuf versions. While there’s no problem with this right now, it does limit us to older libraries and functions and the usual suite of issues with using legacy code.

Unfortunately, the new version significantly restructured the impl for making the low level calls, and will require us reworking our storage code.

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