Allow specifying environment variables for new templates
### Problem to solve
GitLab project templates are great, but limited because they need to be in operational state upon creation (or the experience is weird.) What would be much more powerful is if a new project setup could define questions that set permanent environment variables within the project; this would give template creators the ability to make much easier to use templates.
### Target audience
- Sasha, Software Developer, https://design.gitlab.com/research/personas#persona-sasha
### Further details
More powerful and easier to use templates
### Proposal
To be determined
### Documentation
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### What does success look like, and how can we measure that?
<!-- Define both the success metrics and acceptance criteria. Note that success metrics indicate the desired business outcomes, while acceptance criteria indicate when the solution is working correctly. If there is no way to measure success, link to an issue that will implement a way to measure this. -->
To be determined
### Links / references
issue