Part 1: Abstract pipeline_variables_form to its own component

What does this MR do and why?

Part 1 for #520831 (closed)

This work is behind a feature flag ci_inputs_for_pipelines

We need to abstract the pipeline variables form to it's own component in order to accommodate inputs work and also hide the component depending on settings.

Part 1 simply stubs out the new component and it's test file.

Step	MR
Set up component/query	[Current MR]
Build out pipeline variable form component	!182455 (merged)
Clean up and integration	!182475 (merged)

Screenshots or screen recordings

With the flag disabled, there should be no UI change.

with flag enabled	with flag disabled

How to set up and validate locally

1. turn on the feature flag with Feature.enable(:ci_inputs_for_pipelines)
2. Navigate to Build > Pipeline editor and enter the following into your gitlab-ci.yml

variables:
  DEPLOY_ENVIRONMENT:
    value: "staging"
    options:
      - "production"
      - "staging"
      - "canary"
    description: "The deployment target. Set to 'staging' by default."

job:
  script: echo 1

3. Navigate to -/pipelines/new and verify that your variables are output where the pipeline variables form should be.
4. Turn off the feature flag and insure the variables form and createPipeline mutation work. To verify the variables in the mutation: comment out the line in pipeline_new_form.vue that says visitUrl(pipeline.path), click the "New pipeline" button, and check the network tab.