Fetch statefulset resource to calculate fleet agent status

[torchiaf]

Summary

Fixes #10738

Occurred changes and/or fixed issues

We are adding the fleet's StatefulSet in agent status calculation, for downstream clusters.

Technical notes summary

• local clusters:
  • using both StatefulSet and Deployment
• rke2 downstream clusters:
  • using StatefulSet since the deployment does not exist.

Areas or cases that should be tested

• cluster's dashboard -> healthboxes.

Areas which could experience regressions

To test

See #10614 description.

• there is a known issue when scaling/redeploying fleet StatefulSet; it can be done using kubectl, see Empty serviceName in fleet StatefulSet  #10762

Screenshot/Video

Checklist

• The PR is linked to an issue and the linked issue has a Milestone, or no issue is needed
• The PR has a Milestone
• The PR template has been filled out
• The PR has been self reviewed
• The PR has a reviewer assigned
• The PR has automated tests or clear instructions for manual tests and the linked issue has appropriate QA labels, or tests are not needed
• The PR has reviewed with UX and tested in light and dark mode, or there are no UX changes

[richard-cox]

• shell/pages/c/_cluster/explorer/index.vue fetch loadAgents should not be blocking.