feat: Support global variables/constants

Describe the feature request

I want Unleash to support string variables that can be defined one place, and referenced in multiple activation strategies / constraints / variants. In a string-replacement kind of way.

Background

Purpose is to avoid duplicating configuration, and to easily modify multiple resources safely with less chance of "missing a spot". This is especially useful with larger teams where you want the configurations to be consistent across Projects / strategies.

Solution suggestions

Backend - Variables can be persisted in postgres database. Frontend - How it could look using $(VAR_NAME) syntax :

Variables page

Constraints page:

Variants page: