Support spec::inputs for pipelines

[KahlerTak]

Is your feature request related to a problem? Please describe.
We started using inputs in our project pipelines, to make use of their advantages over usual variables. However, it is now harder to test the pipeline code, since we cannot use this awesome tool anymore or only in parts.

Describe the solution you'd like
It seems, that this feature is basically a templating engine befor the actual start of the pipeline. I would assume it is based on some kind of known templating engine, though I couldn't find which one exactly.

Describe alternatives you've considered
I considered using a own template engine, to at least be able to use my scripts for basic configs, but like I said, I wasn't able to finde the correct syntax for the engines known to me and gave up pretty fast.

Additional context
The spec:inputs is very useful for variables effecting pre runtime stuff like generating job names and including the correct files. We currently use it mostly for different builds, like nightlies and dev builds. It's very convenient in those cases.

[firecow]

What have you tried?

Provide a very very simple example, that we can use to reproduce please, and also the cli commands that you are using to test your "spec::inputs".

[KahlerTak]

I pushed the example right here:

https://github.com/KahlerTak/gitlab-ci-local-tests

Creating it, I realized there is some support for spec::inputs, as defaults are always used as the values. Must have never tried that before. However, it would be nice to support a .gitlab-ci-local-variables like syntax for them as well. But maybe I'm missing something?