Add an optional caption column to the Frontify asset field

Problem/Motivation

The Frontify asset field stores name/alt data but has no per-asset caption, so sites that render captioned figures must bolt a separate field next to every Frontify field and keep the deltas in sync manually.

Proposed resolution

Add an optional caption column to the field schema, expose it on the widget edit sub-form, and persist it through the save mappings (including extracting a sensible default from the asset's Copyright metadata when available).

We run this in production as a composer patch against 3.0.1 (schema + widget + save mappings) on Drupal 11 / PHP 8.4. Happy to open an MR if the approach is agreed.