Add caching for client credentials token

[rudsi]

Closes the issue #10422

This PR introduces a thread-safe, in-memory cache for the OIDC client credentials access token.

Previously, a new token was fetched via a network call for every protected outbound request, causing unnecessary latency. This change implements a double-checked locking pattern to ensure a token is fetched only when the cached one is missing or expired.

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[rudsi]

Hi @botsteve, I’ve opened a PR for the issue. Please review it when you get a chance and let me know if any changes are needed. Thanks!

[rudsi]

Hi @spericas, when you have a moment, could you please review this PR for me? Thanks!

[rudsi]

Hi @spericas, resolved checkstyle errors, could you please approve the workflow.

[romain-grecourt]

Please try to avoid (re)formatting code that you aren't modifying.
This will remove the noise from your diffs and make it easier to review your changes.

[rudsi]

Hi @romain-grecourt,
The reformatting in the previous commit was a result of my IDE settings—I'll make sure to avoid that in the future.

I've addressed the bugs which were causing the build checks to fail. When you have some time, I’d appreciate it if you could review the latest changes.

Thanks!