Connector CI: Use one service account

[bnchrch]

What

A demo of what using one service account would look like

TODO before merge

1. Give the github ci SA admin access to spec cache
2. Give thegithub ci SA admin access to the metadata service

closes: airbytehq/airbyte-internal-issues#1850

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[alafanechere]

To move forward on this PR we'd need the global SA to be created first right?

[alafanechere]

IMO the env var / SA should be named like GCP_CI_CREDENTIALS and would have permissions to:

• Read GSM secrets
• Write to spec cache bucket
• Write to metadata bucket
• Write reports

I also believe these permissions should be mapped to role, and we can create different SA/users with this role. We can have 1 SA for the CI pipeline. We could decide to give a pool of gcloud users (dev conectors ops or full connector org team) the permission to use this role so they can run easily pipelines locally with the same kind of operations that happen in the CI.