Draft: POC: Expand job inputs into the user script

What does this MR do?

This MR is a spike to see how job inputs could be replaced in a user script.

This MR should not be merged, the implementation is just enough to get the functionality working. Secrets are not masked in this POC implementation.

How to run the POC

1. From the most recent MR pipeline, find the binaries job. Download the gitlab-runner-[os]-[arch] binary for your platform.
2. Make sure the binary is executable, chmod +x [gitlab-runner-os-arch]
3. Create a test project
4. In the project Runner settings, create a project runner with a custom tag
5. Run the register command the UI shows using the gitlab runner binary you downloaded
6. Run gitlab-runner-[os]-[arch] run
7. In the test project, run a pipeline that has a job tagged with the custom tag

Reference

PoC: Add job inputs interpolation to runner (gitlab#542425 - closed)