Merge branch '520831-pipeline-variables-boilerplate' into 'master'

Part 1: Abstract pipeline_variables_form to its own component

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/182376



Merged-by: Laura Montemayor <lmontemayor@gitlab.com>
Approved-by: Laura Montemayor <lmontemayor@gitlab.com>
Co-authored-by: Briley Sandlin <bsandlin@gitlab.com>