support for exteral member validator manager

[havetisyan]

Description

introduce an external member validator manager responsible for managing all domains that are configured with external member validator and responsible for calling the checks #3255

Contribution Checklist:

• The pull request does not introduce any breaking changes
• I have read the contribution guidelines.
• Create an issue and link to the pull request.

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[gemini-code-assist]

Summary of Changes

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This pull request introduces a new capability to Athenz ZMS (ZMS) by implementing an "External Member Validator Manager." This manager allows domains to define custom logic for validating members, which can be dynamically loaded and refreshed. This enhances flexibility by enabling external systems to enforce specific membership criteria beyond Athenz's built-in validation, ensuring that only authorized and compliant members can be added to roles within configured domains.

Highlights

• External Member Validator Manager: Introduced a new manager responsible for dynamically loading and managing external member validators for domains, enabling custom member validation logic.
• Database Integration: Added a new method to the ObjectStoreConnection interface and its JDBCConnection implementation to retrieve domains configured with external member validators from the database.
• ZMS Service Layer Update: Integrated the new database functionality into DBService to fetch external member validator configurations.
• Scheduled Refresh: The ExternalMemberValidatorManager includes a scheduled task to periodically refresh the list of active external member validators from the database.
• Configuration: Added a new system property (athenz.zms.external_member_validator_frequency_hours) to configure the refresh frequency of the external member validators.
• Comprehensive Testing: Included extensive unit tests for the new ExternalMemberValidatorManager and the updated JDBCConnection and DBService methods to ensure correctness and robustness.

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Code Review

This pull request introduces an External Member Validator feature to Athenz ZMS. It adds a new interface method to ObjectStoreConnection and its JDBC implementation to retrieve domains configured with external member validators from the database. A new ExternalMemberValidatorManager class is introduced to dynamically load, manage, and refresh these validators using a scheduled executor, and to validate members against the configured external logic. Corresponding DBService methods and configuration constants have been added, along with comprehensive unit tests for the new functionality. Review comments suggest making the ExternalMemberValidatorManager class final to prevent this escaping, providing a custom ThreadFactory for better thread naming, and improving exception handling and logging during validator instantiation.