Added support for x509 client authentication

[Thendo20]

What this PR does / why we need it:

This PR adds support for X.509 certificate-based client authentication to keycloak-config-cli. Users can now authenticate to Keycloak using mutual TLS (mTLS) with client certificates instead of only username/password or client credentials (client_id/client_secret).

Key Features:

• New RestClientX509TokenManager for managing token acquisition via certificate authentication
• Configuration support for keystore and truststore with certificate credentials
• Automatic SSL context configuration for client certificate authentication

Use Cases:

• Enhanced security in production environments requiring certificate-based authentication
• Compliance with security policies mandating mTLS
• Integration with PKI infrastructure
• Zero-trust security models

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #959

Special notes for your reviewer:

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud