Ability to use mock Terragrunt outputs as mock stack dependency outputs

Currently outputs from stack dependencies can’t be accessed by a dependent stack until a tracked run happens at least once. Terragrunt has a concept of mock outputs for dependencies for this reason.

Spacelift should add the ability to add mocks for dependency outputs on a stack, so that a chain of dependencies can be planned together before merging them.

Ideally they would be able to utilize the Terragrunt mock outputs, and use those as the dependency outputs until the first tracked run happens. Even better would be if a dependent stack would use the outputs of a stacks proposed run, if the branch names matched.