Simulate pipeline from the pipeline editor

What does this MR do and why?

For #337282 (closed)

This adds functionality to the Validate tab of the pipeline editor. From here, users can run a pipeline simulation with the content of their default CI configuration.

If the user edits their config, they can re-run a new simulation. If they cancel the simulation, the Validate tab goes back to the empty state.

Feature Breakdown

frontend	MR
Introduce feature flag to replace Lint tab with Validate tab in the pipeline editor	!89497 (merged)
Add popover and NEW badge to the Validate tab	!90345 (merged)
Add simulation functionality to the Validate tab	👈 you are here
Roll out feature flag	#364257 (closed)

Screenshots or screen recordings

Behavior	Recording
Pipeline simulation with changing CI content	Screen_Recording_2022-07-05_at_19.35.13
Canceling a simulation	Screen_Recording_2022-07-12_at_18.01.00

How to set up and validate locally

1. Enable the simulate_pipeline feature flag. (rails c > Feature.enable(:simulate_pipeline))
2. Go to the pipeline editor. (CI/CD > Editor)
3. Go to the Validate` tab and run a pipeline simulation. Verify the results.
4. Go to the Edit tab and edit the file to have an invalid CI configuration.
5. Go back to the Validate tab and re-run the pipeline. The results should reflect the new content.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.