feat: Add ui & api for exposing controlplane cert expiry

[jihyun-huh]

Quick Summary

add Exposing controlplane's certificate expiry information on Azimuth UI & API
The Azimuth API now reads the certificate status published by azimuth-capi-operator, and the UI displays:

• the earliest known controlplane certificate expiry date
• control_plane_certificate_expiry_date: expected date of machine's cert expiration
• control_plane_certificate_rotation_days: number of spec.rollout.before.certificatesExpiryDays on KCP
• control_plane_certificate_rotation_date: calculated auto replacement date with control_plane_certificate_expiry_date and control_plane_certificate_rotation_days

the fields would be hidden when certificate information is unavailable, and this is visibility-only implementation for exposing the information only. no manual renewal action button is included.

Testing

• added API conversion tests for present and missing certificate status
• testing api & ui on ha management cluster of leafcloud

AS-IS vs TO-BE

AS-IS (current)

TO-BE-0

(if KCP.spec.rollout.before.certificatesExpiryDays is none)

• only expose certificate expiry day

TO-BE-1

(if KCP.spec.rollout.before.certificatesExpiryDays is set)

• expose certificate expiry day, expected auto replacement day, and auto-replacement notification message below

[amanning9]

As discussed some suggestions to make the UI a little more intuitive

[amanning9]

I think something like this makes it a little more intuitive:

<tr><th class="align-top">Kubernetes API Certificate</th><td>
  Expires 4 October 2026
  <br>
  <span class="small">
    will renew automatically on 13 Sept 2026
  </span>
  <br>
</td>
  <td>
    <button type="button" class="me-2 btn btn-warning">Renew Now</button>
  </td>
</tr>