Add support for PHP 8.1 Backed Enums in Select, Checkboxes, Radio elements

Problem/Motivation

PHP 8.1 comes with support for Backed Enums
It makes sense to be able to use them for select, radio and checkboxes fields as the value for #options

Proposed resolution

Support using an Backed Enum for #options

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet