ft(pipeline): Add variables flag for pipeline run

[zemzale]

Description
This adds flag for running pipelines with custom variables passed to it.
They all default to env_var, for simplicity.

Related Issue

#319

Motivation and Context

This allows to run pipelines from command line with custom variables.
How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[zemzale]

@profclems I wrote up this MR so we can add this feature. Look over it please comment if there are some problems you see.
I intend to add some tests for this but I don't have the time for it right now, but would love some feedback so I can know that this is going the right way.

[profclems]

I'm okay with the implementation. Looks clean 🎉

[profclems]

@profclems I wrote up this MR so we can add this feature. Look over it please comment if there are some problems you see.
I intend to add some tests for this but I don't have the time for it right now, but would love some feedback so I can know that this is going the right way.

I actually like the implementation. This looks good. You can add the tests in a separate PR when you're ready

[zemzale]

Yeah looks like I might add tests later on. I might not have the time in the next days, and never had done testing for CLIs like this so it's a bit to wrap my head around. If everything is okay then you are free to merge this