Add control for masking variable values in runner logs

What does this MR do?

❗ Minimal version:

• Add a "Masked" toggle to the Variables section
• Check values against regex/rules that check if the value is suitable for masking
• Still allow user to save values that don't match, as long as they've been warned
• Update paragraph above to explain how this works

🌟 Ideal version here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25540

"Suitable for masking" means:

• it should be a single line
• it should not have escape characters
• it should not use variables
• it should not have any whitespace
• it should be longer than 8 characters

Screenshots

What are the relevant issue numbers?

#59318 (moved), closes #13784 (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.
• Security reports checked/validated by reviewer