[Gemini] Add compatibility classes between apitools client and modern bq client

[jrmccluskey]

Creates compatibility shims between generated BigQuery classes and the modern google-cloud-bigquery client to be used in migrating support over to the modern client.

Master PR - #39889

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[jrmccluskey]

R: @damccorm

This should be good to take a run at, had to track down a docs precommit problem. Going to put improving the readability of the output of that test suite on my to-do list.

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[damccorm]

Left a couple of preliminary comments to make sure I'm understanding the approach broadly, thanks

[damccorm]

If apitools is imported, will we ever need this fallback? Same thing applies elsewhere

[damccorm]

Who do we expect to need this fallback? IIUC, it would just be people who depend on the internal generated BQ client, but we don't expect "normal" users to use it. Is that right? If so, I think we should force users to opt in to the fallback somehow - this will both alert them that the deprecated module will eventually go away and that they're in a potentially unsafe mode.