Hard dependency on ADFS module in Admin Federation page #2674

This is a reminder issue to move away from hard dependency on ADFS module in modules/admin/src/Controller/Federation.php. The idea is to abstract this away using events.

See

• Fix admin federation page calling a removed SAMLBuilder method for adfs #2672 (comment)
• Resolve hosted metadata endpoints in the module simplesamlphp-module-adfs#51 (comment)