Store Keepeek Asset ID as hidden field value

Problem/Motivation

A separate feature request will add an API endpoint to allow Media Assets to be updated or deleted. To facilicate this, the Asset ID will be to be stored as a field level value to allow an Entity Query to be run, to find matching Drupal media items.

Proposed resolution

Add the hidden field.
Add an update hook to loop through existing Keepeek Media items and pull the value from the json field and popupalate the field.

Remaining tasks

Write the code.

User interface changes

n/a

API changes

n/a

Data model changes

Additional field