Manage LDAP phone extension attribute

[DavidePrincipi]

Manage the LDAP phone extension attribute from cluster-admin. Display it read-only for remote account providers.

Proposed solution

• Allow to modify the LDAP phone extension attribute for both AD and OpenLDAP providers
• Modify the attribute from both cluster-admin and user-portal UI

Alternative solutions

None

See also

• Discussion https://mattermost.nethesis.it/nethesis/pl/fnj4epp9kf8ffe3fa18cm9x5zr
• Discussion https://mattermost.nethesis.it/nethesis/pl/w95t1ufqztd1xqa1u8jtqgy9qo