Make Stack vendor version constraints more flexible

Currently you can define the used version for HashiCorp Terraform / OpenTofu in your Stack configuration with SemVer. This works fine if you are managing your Stack through .spacelift/config.yml in the same repository, but if you manage your Stacks through the Terraform provider you end up with difficulties testing:

1. Update the used version in the parent Stack

2. Test the newly used version in the child Stack

3. If good, merge 1 and 2.

Ideally the version constraint field would work similar to tenv where you would only need to specify the used tooling (HashiCorp Terraform vs OpenTofu) and it would pick the required version based on your code.