✨ Add `Set-AzDevOpsGroupVariable` for more fine-grained control of the DevOps variable group

[stijnmoreels]

Is your feature request related to a problem? Please describe.
We currently have a Set-AzDevOpsArmOutputsToVariableGroup that provides a way to place all output variables of a ARM/Bicep deployment to an Azure DevOps variable group. But for a more fine-grained control on what needs to be set - and to also set variables that aren't always outputed, there is no easy way to add such a variable.

Describe the solution you'd like
Adds an accompanied Set-AzDevOpsGroupVariable that allows us to set a single variable in an Azure DevOps variable group.

Describe alternatives you've considered
People are forced to output everything they need, even when that information is not applicable or related to the ARM/Bicep's components. That's how it's currently done/available to do.

Additional context
In the Arcus.Scripting.DevOps module.

[fgheysels]

Is this still relevant, as the related PR has been closed (without being merged).

[stijnmoreels]

PR closed as the DevOps Practice might take another route, issue still open bc the Practice might need some additional input to propose alternatives.