WIP: Add new options to the "Settings > General > Merge requests" page for Prospective Merge Pipelines

What does this MR do?

Updates the Settings > General > Merge requests page to include configuration options for the Prospective Merge Pipelines feature.

This MR only adds the options in the UI; it does not add the new options to the underlying data structures.

Relevant UX Spec: https://gitlab-org.gitlab.io/gitlab-design/hosted/dimitrie/ee%237380-run-build-on-merged-code-spec-previews/#artboard5

What are the relevant issue numbers?

#7380 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer