Add initialization step

[luca-della-vedova]

Certain downstream repos might need an initialization step to be ran before the workflow itself.
Reusable workflows cannot be executed as a step so adding a step in the downstream repo is not possible (I believe?), so I tried the approach of allowing command injection by specifying an input variable that is executed as a bash command. The step is ignored if no command is specified.

Example of usage in colcon/colcon-ros-cargo#22. colcon-ros-cargo needs to run cargo install cargo-ament-build before it can run its CI, so we inject it into the prerun-step variable and that allows us to setup the workflow correctly.

Note that I'm not sure there is a better way to add a setup step before running a reusable workflow, I saw threads suggesting using a local action but didn't have much luck with it, open to suggestions if there are better ways!

[jhdcs]

I feel like if this works for now, we might as well add it in. No need for perfection to become the enemy of good enough.