WIP - Refactor custom fields handling for storage

[snipe]

This just condenses down the custom field stuff into a smaller method. @uberbrady - I know you were working on something similar for the upcoming custom fields for users, but this should be a nicer way for us to bridge that gap. I just need to make sure the additional tests are there. We do something slightly different with bulk updating assets, since we're passing it an updated array there, so I didn't touch that for now.

[what-the-diff]

PR Summary

• Implementation of New Method for Custom Fields
  We have introduced the handleCustomFieldsForStoring method in the Asset model. This new method is designated to manage updates to the custom fields.

• Consolidating Code in AssetsController
  We have streamlined the code in both Api/AssetsController.php and AssetsController.php files. The sections dealing with the update of custom fields in store and update methods have been replaced with a call to the new handleCustomFieldsForStoring method. This results in cleaner and more efficient code.

• Inclusion of Additional Facades in Asset model
  We have included usage of Illuminate\Support\Facades\Crypt and Illuminate\Support\Facades\Gate in the Asset model. They provide important functionalities necessary for encryption processes and authorization checks.

[snipe]

Tests failing here (only on Sqlite) - looking into it