feat: add additional gateway_manager services

[privateip]

• Replace Mapping parameters with explicit keyword arguments in create/update methods
• Add create_gateway with required and optional parameters (cluster_id, name, description, enabled, certificate_id, connection_config, tags, metadata)
• Add update_gateway with explicit optional parameters for partial updates
• Add update_certificate with explicit optional parameters (alias, contract_id, raw_certificate)
• Add create_service_group with explicit parameters
• Add update_service_group with explicit optional parameters including add_services and remove_services
• Build request bodies conditionally to exclude None values