feat: Improve config validation with JSON schema

[Motouom]

What this PR does / why we need it:
This PR introduces comprehensive configuration validation using a JSON schema, significantly improving the developer experience and ensuring the correctness and reliability of Keycloak configurations. By integrating a robust schema, we provide real-time feedback in IDEs, enable automated validation in CI/CD pipelines, and streamline the code review process for configuration files. This reduces errors, enhances maintainability, and clarifies expected configuration structures.

Which issue this PR fixes: fixes #1122

Special notes for your reviewer:

This PR integrates an external JSON schema for Keycloak import configurations, adapted from jwesselink/keycloak-import-schema. It includes updates to example configuration files (benchmark.yaml, userprofile.yaml) to conform to the new schema, improved boolean validation messages within the schema itself, and new documentation (docs/SCHEMA_USAGE.md) detailing how to leverage this schema in various development stages (IDEs, CI/CD, code reviews). Please pay close attention to the schema's structure and the updated example files to ensure they meet the project's standards.

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 failed

Failed conditions
B Maintainability Rating on New Code (required ≥ A)

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