Metadata for categories in the Headless API

It would be useful to expose a metadata field for categories in the Headless API, similar to the recently added metadata support for packages.

Use Case:
Developers may want to extend the options provided by the default category editor. For example, metadata could be used to define a background colour for a category card, specify how packages within a category should be displayed (grid, list, table, etc.), or provide other category-specific settings.

Adding metadata support for categories would give developers greater flexibility when building custom storefronts, while still allowing Tebex to act as the sole CMS for their webstore. Reducing the need of maintaining a custom API, or configuration system.