Add user preference for build timestamp format

[somiljain2006]

Fixes ~#16804

This change introduces backend support for a user preference that controls build timestamp formatting by adding a UserProperty descriptor and a fillItems endpoint. The backend implementation is complete and verified. However, the preference is not yet reflected in the UI due to Jenkins core JavaScript behavior issues encountered during development, despite multiple attempts to integrate it. A separate follow-up issue can be raised to track the UI-side work.

Steps to verify it:-

1. Run the war file locally
2. Go to this link: http://localhost:8080/descriptorByName/jenkins.model.DateTimePreference/fillTimeFormatItems
3. Confirm that the endpoint returns a ListBoxModel containing all supported timestamp format options.

Testing done

Ensured all tests still passed.

Proposed changelog entries

Internal support for configurable build timestamp formatting.

Proposed changelog category

/label internal

Proposed upgrade guidelines

N/A

Before the changes are marked as ready-for-merge:

Maintainer checklist

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
• Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
• Proper changelog labels are set so that the changelog can be generated automatically.
• If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set, and there is a Proposed upgrade guidelines section in the pull request title (see example).
• If it would make sense to backport the change to LTS, be a Bug or Improvement, and either the issue or pull request must be labeled as lts-candidate to be considered.

[comment-ops-bot]

I wasn't able to add the following labels: none

Check that the label exists and is spelt right then try again.

[daniel-beck]

A separate follow-up issue can be raised to track the UI-side work.

No. This is incomplete and will not be accepted as is.