Handle rollback command/action in upgrade flow

[pchila]

This issue implements the main part of the manual rollback feature.

When the agent receives a manual rollback request via an action or an elastic-agent upgrade command it should:

• enter the upgrade entrypoint with a flag indicating that this is a rollback request
• skip all the download, extraction, copying of agent artifacts, creation of the upgrade marker
• verify that the state is valid for a rollback:
  • there's an upgrade marker with available_rollbacks populated
  • the upgrade state is valid: UPG_ROLLBACK_AVAILABLE or UPG_WATCHING (if Allow elastic-agent rollback in UPG_WATCHING state #6890 has been implemented already)
• write (safely, using the necessary file locks) desired_outcome: ROLLBACK in the upgrade marker
• start a watcher in case one isn't already running.