VTex Get subcollection by collection ID

>⚠️ There are two ways to configure collections, through Legacy CMS Portal or using the Beta Collection module. This endpoint is compatible with [collections configured through the Legacy CMS Portal](https://help.vtex.com/en/tutorial/adding-collections-cms--2YBy6P6X0NFRpkD2ZBxF6L). Retrieves all subcollections given a collection ID. A subcollection is a [Group](https://help.vtex.com/en/tutorial/adding-collections-cms--2YBy6P6X0NFRpkD2ZBxF6L#group-types) within a collection.