[Feature Request]: Use BigQuery emulator

[thomas-flynn-ingka]

What would you like to happen?

I want to have a purely local development environment.
There is a BigQuery emulator (https://github.com/goccy/bigquery-emulator) with a docker image freely available (ghcr.io/goccy/bigquery-emulator:latest).
How do I change the googleapis endpoint that beam points at?
Is there an environment variable to do this?

This will be used in a continuous integration environment rather than connecting to a google environment.

Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

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[kennknowles]

Merging with #28149 28149