Assign an agent to an ISO.

If the agent is on another ISO, they move to this one and the change is kept in their history. Assigning an agent to the ISO they are already on does nothing.

Errors:

• 409 ISO_INACTIVE if the ISO is deactivated.
• 409 ISO_TARGET_NOT_AGENT if the user isn't an agent.
• 409 ISO_CAPACITY_EXCEEDED if the ISO already has the tenant's maximum number of active agents.
• 409 AGENT_BLOCKED_IN_TENANT if the person is blocked in the tenant. Root can still assign them.
• 404 if the user is in another tenant.

Available to tenant owners and admins.