[GMail] Validate Google Directory auth

[timgrein]

Related to https://github.com/elastic/enterprise-search-team/issues/5592

This PR adds validation logic to check, whether the provided service_account_credentials, customer_id, subject values and the OAuth2 scopes are correctly setup, so the GMail connector can authenticate against the Google Directory API.

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Pre-Review Checklist

• this PR has a meaningful title
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• if there is no GH issue, please create it. Each PR should have a link to an issue
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[artem-shelkovnikov]

Curious - is there a way to make this error message more specific? E.g. "You're missing scope X" or similar?

[timgrein]

Curious - is there a way to make this error message more specific? E.g. "You're missing scope X" or similar?

There's no "official" recommended way AFAIK. This API can probably return the scopes, I'll look into it and create a follow-up, if it's successful.

[github-actions]

💔 Failed to create backport PR(s)

The backport operation could not be completed due to the following error:
There are no branches to backport to. Aborting.

The backport PRs will be merged automatically after passing CI.

To backport manually run:
backport --pr 1565 --autoMerge --autoMergeMethod squash