OIDC: document and use OidcTestClient to get access token from Dev Services for OIDC

[michalvavrik]

I realized when working on other PR that tests I added for Dev Services for OIDC should use OidcTestClient to get bearer access token. I also added docs for someone else to mention it as well. Docs addition is not important, I can drop the docs if you deem it useless.

[github-actions]

🙈 The PR is closed and the preview is expired.

[quarkus-bot]

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[quarkus-bot]

Status for workflow Quarkus CI

This is the status report for running Quarkus CI on commit e227b9c.

✅ The latest workflow run for the pull request has completed successfully.

It should be safe to merge provided you have a look at the other checks in the summary.

You can consult the Develocity build scans.