Elastic agent should add an available rollback entry in update marker if `rollback_window` is set

[pchila]

Requires #6881

GIVEN an upgrade has been triggered from <version A> to <version B> at a time t0
WHEN Elastic Agent creates the update marker AND
rollback.window > 0
THEN newly created update marker contains an entry under rollbacks_available similar to

details: 
  target_version: `<version B>`
  metadata: {}
 rollbacks_available:
      - version: `<version A>`,
        home: data/elastic-agent-<Version A>-<Version A commit hash>
        valid_until: "<t0 + agent.settings.upgrade.rollback.window>"

GIVEN an upgrade has been triggered from <version A> to <version B> at a time t0
WHEN Elastic Agent creates the update marker AND
rollback.window == 0
THEN newly created update marker will not contain an entry under rollbacks_available

[elasticmachine]

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

[pchila]

rollback_window config will be used by the elastic-agent main process to create the rollback_available field when first writing the marker, so no need to pass the parameter to the watcher

[ebeahan]

Current status - pending the work @pchila iterating on in #8767.