Add rollback boolean field to ActionUpgrade

[pchila]

Add a rollback boolean field (defaults to false) in upgrade action and propagate it to elastic-agent upgrade entry point.
The new upgrade action schema should look like:

actionUpgrade:
  description: the UPGRADE action data.
  type: object
  required:
    - version
  properties:
    version:
      description: The version number that the agent should upgrade to.
      type: string
    source_uri:
      description: The source of the upgrade artifact.
      type: string
    rollback:
      description: Indicates if this version change should be performed as a rollback to a previous version.
      type: boolean

[elasticmachine]

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