feat: Support global variables/constants

[Hansemeister]

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:

[FredrikOseberg]

@Hansemeister Thanks for the suggestion! I think we already have something in this area that may make it easier for you to manage configuration for multiple feature toggles. For this purpose we already have segments which allows you to define re-usable sets of constraint that can be updated in one central place. Does that solve this use case for you?

[Hansemeister]

@FredrikOseberg Neat! Unleash Segments solve my use case for the constraints.
However for the variants, it could still be useful to have variables to use in the payloads.