Cache access token for client credentials flow

[botsteve]

Description

Documentation

• Uses a single CachedToken instance to cache the client credentials access token per provider/config, not per outbound target or tenant.
• Access to the cache is protected by a ReentrantLock for thread safety.
• If the OIDC server does not provide expires_in as it is a RECOMMENDED but not REQUIRED field in the response https://datatracker.ietf.org/doc/html/rfc6749, the caching is skipped in order to avoid scenarios where you get stuck with an expired cached access token.
• The CachedToken class uses a 30-second buffer to refresh the token before it actually expires.

Closes the issue #10422

[botsteve]

@romain-grecourt or @Verdent if you can review sometime. Thanks!

[botsteve]

Hei @Verdent, Happy New Year! Can you spare some time to merge this if it's ok?