Add `desired_outcome` field to upgrade marker

[pchila]

Elastic Agent will write a new desired_outcome field in the upgrade marker to indicate if the ongoing upgrade is to be completed or rolled back as requested by the user.

The values for desired_outcome can be:

• UPGRADE
• ROLLBACK

The initial value UPGRADE is written when the marker is created first and it will be changed to ROLLBACK when the agent receives a manual rollback request.

Elastic Agent watcher must proceed to rollback and cleanup the new agent installation as soon as the desired_outcome is set to ROLLBACK.

Definition of done:

• When a new upgrade marker is created, it contains a desired_outcome with value UPGRADE
• Include support for ROLLBACK value of desired_outcome needed for implementing Handle rollback command/action in upgrade flow #6889

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)