Fixed sorting on last checkin assets api

[snipe]

This fixes a few missing elements from the change in #14262, namely the ability to sort and search on the last_checkin value that was added to the UI.

[what-the-diff]

PR Summary

• Enhanced Asset Details Visibility
  The latest update in 'AssetsController.php' includes 'last_checkin' in the attribute list. This addition ensures that users can now see the last time an asset was checked-in, as part of standard asset info.

• Validation Update for Checkouts
  In 'Asset.php', the validation rules for the 'declinedCheckout' method have been updated to include 'last_checkin'. This ensures that checkouts are tightly controlled and accurate information is maintained.

• Improved Mass Assignment
  The 'last_checkin' attribute has also been added to the list of attributes that can be filled through mass assignment. This makes data management of asset check-ins easier and more efficient.