Add ability to provide the agent-id in enroll API

[blakerouse]

Describe the enhancement:

Add the ability to specify the agent-id of the enrolling Elastic Agent.

Describe a specific use case for the enhancement or feature:

On serverless, an Elastic Agent is static but the pod doesn't have any persistent storage so it cannot store the enrollment information between restarts of the Elastic Agent. There have also been other reports of this issue from customers where they do not need persistent storage from the integration and requiring the Elastic Agent to have it just for the enrollment information is not possible.

To provide a stable Elastic Agent in the agents list in Kibana, this would allow an Elastic Agent to enroll with the ID they want to have. This would also replace an existing Elastic Agent if one already has the same ID.

This does open the possibility that two Elastic Agent's are communicating to Fleet Server as the same agents. This is acceptable for a moment in-time, like when a new pod rolling out before the old pod is stopped. Running two concurrent Elastic Agent's at the same time would be possible, and would result in undefined behavior and would not be recommended.

[michel-laterman]

I think we already provide this through the enrollment_id in the API:

fleet-server/model/openapi.yml

Lines 150 to 155 in 0687ea5

	enrollment_id:
	type: string
	description: |
	The enrollment ID of the agent.
	To replace an agent on enroll fail.
	The existing agent with a matching enrollment_id will be deleted if it never checked in. The new agent will be enrolled with the enrollment_id.

It was added with #2655

[blakerouse]

@michel-laterman The existing agent with a matching enrollment_id will be deleted if it never checked in. What if it has checked-in?