id widget

**Is your feature request related to a problem? Please describe.**

Whenever you model a collection you need an identifier field for its entries. Most of the time the collection does not have a natural identifier field (i.e. a primary key in RDBMS parlance). For this reason the community developed a range of custom widgets to fill this hole in the NetlifyCMS standard widget library, that does not offer such a basic widget.

**Describe the solution you'd like**

Create an `id` widget to generate an universal unique id code (i.e. UUID). This widget should also have a boolean `hide` option to allow the associated `field` to be hidden from the UI.

**Describe alternatives you've considered**

Use a community based custom widget, but none of them currently allows to hide the field from the UI.

**Additional context**

Anyone intending to migrate a three-tire web application to JAMstack architecture will need to model collections and collection relationship. And you can't do this without having an `id` widget resembling synthetic primary key in RDBMS parlance.