Add custom field widget definition to all work item types

What does this MR do and why?

This adds the custom field widget definition to all work item types.

This is currently in development and will only be returned to the frontend when the custom_fields_feature feature flag is enabled.

References

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• #504532 (closed)

MR acceptance checklist

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How to set up and validate locally

1. Enable the custom_fields_feature feature flag

2. In the GraphQL explorer, try a query like:

   query {
     namespace(fullPath: "flightjs/flight") {
       workItem(iid: "81") {
         id
         title
         widgets {
           type
         }
       }
     }
   }

It should include the custom fields widget in the list