[Fleet] Expose method to notify fleet of policy changes

[mattapperson]

method sig: policyUpdated(policyId: string, type: "added" | "updated" | "removed", payload: Policy | null): Promise<{success: bool}>

Current use-case is limited to type "removed", where we should unassign all agents with the provided policy ID

[nchaulet]

What the status of an Unassigned agent, we still agent enrolled but no policy returned during checkin? it's something we want to display in the UI, with a new status?

[mattapperson]

Great question.
For now such agents should be unenrolled.

If we/when we support moving agents between policies in the UI we will set a status of
"Disabled" / "Unassigned"