feat: Add resilient, non-blocking realm registration for Token Status plugin

[Awambeng]

This PR improves the reliability and performance of realm registration in the Keycloak Token Status Plugin by making it asynchronous, non-blocking, and more fault-tolerant.

Key Improvements

Non-blocking registration

• Startup and lazy registration now run in background threads
• Prevents Keycloak boot delays and OIDC request timeouts

Self-healing behavior

• Automatically triggers registration on the first OIDC request if not yet completed
• Adds a 60s cooldown per realm to avoid repeated failures when the server returns errors

Improved network resilience

• Retry mechanism with exponential backoff (1s, 2s, 4s, …)
• Increased timeout to 30s for slow responses

Stability fixes

• Fixed NPE during master realm initialization
• Added thread-safe per-realm locking for concurrent requests

For more details see: https://adorsys.atlassian.net/wiki/x/2IoXl

Closes #69

[forkimenjeckayang]

A few concerns to check @Awambeng
Also for CustomHttpClient.java StatusListConfig.java

The new defaults (retry=1, timeout=30s) are reasonable for startup robustness, but this client is reused in runtime request paths too. Can we split startup vs runtime HTTP policies (or make retry configurable) to avoid increasing tail latency for user-facing flows?
WDYT?

[IngridPuppet]

I left some comments. Please could you check?

[IngridPuppet]

LGTM

[forkimenjeckayang]

No further concerns