Refactor Bigtable impl into a support.rs

[data-sync-user]

The bigtable_client root module (mod.rs) became very large as we prototyped it: it can be easily split into smaller modules. One obvious separation point is the BigTableClientImpl code (which is largely generic Bigtable API calls) vs the impl DbClient – really our actual client implementation on top of the generic calls.

We could split it into at least a support.rs and possibly into further separate modules.

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