Allow only a group of agents to have a assign permission

[Shindhu-Ramaswamy]

Currently, any user can assign or remove assignees from a ticket.
There is no restriction based on roles.

This can cause issues because:

Unauthorized users can change ticket assignments
No proper control over who can manage tickets

For that i came up with the solution:

I added a role-based check to control ticket assignment.

Backend
Created a method can_assign_ticket() to check user roles
Allowed roles:
Ticket Assigner
Agent Manager
System Manager
Added validation in remove_assignments:
If user does not have permission → throw error
Frontend
Updated AssignTo.vue
If user has permission:
Show assign UI
If not:
Show assignee list as read-only

#issue #2488