feature: Tenants management role

[anarsultanov]

This PR introduces tenants management role allowing administrators to list and manage all tenants. The implementation builds upon Keycloak's established authorization patterns and provides a clean, efficient solution while preserving backward compatibility.

Closes #48

Backward Compatibility

• No impact on existing tenant member functionality.
• The new role introduces additional capabilities without altering existing flows.

Setup Instructions

To enable this feature:

1. Create the Role

   • Navigate to Clients > realm-management > Roles in the Keycloak Admin Console.
   • Add a new role: manage-tenants.

2. Assign the Role

   • Assign the manage-tenants role to realm administrators as required.

Thank you to @oleaasbo for the detailed proposal.