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

**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.

Work is in the Arcus.Scripting.DevOps module and should complement Set-AzDevOpsArmOutputsToVariableGroup. Start by locating that existing command and its Azure DevOps variable-group interaction. Done means a companion Set-AzDevOpsGroupVariable can set one specified variable without requiring it to be an ARM/Bicep output, with the module's relevant checks updated.

Written by the indexing model from the issue text.