MLC-296/complete implementation of agent management module

• Implemented agent creation with validations (email, first/last name, phone, role, agency)
• Validated agent data using centralized ValidationUtils
• Integrated Keycloak user registration during agent creation
• Implemented agent update including address and Keycloak sync
• Added agent status change and deletion features
• Developed paginated and filtered agent search (name, CIN, email, phone, role, status, dates)
• Integrated with Agency microservice to fetch agency name by ID
• Centralized search logic in SpecificationUtils
• Added support for retrieving agent by ID
• Implemented isAgentExists(Long id) method
• Created getManagerById and getAllManagers methods using ManagerResponseDTO (with @Builder)