Resolve "Typed AWS environment variables for access keys & region"

What does this MR do?

In order to add an autocomplete list and validations for a limited set of AWS keys, this MR:

• Adds a combobox component to the CI/CD variables modal. (Followup issue to abstract to Gitlab UI ▶ gitlab-ui#716 (closed))
  • Typing matches tokens; dropdown disappears when there is no match
  • Up and down arrows navigate the dropdown
  • Enter selects keyboard-highlighted item; clicking overrides this
  • Esc closes dropdown when it is open, clears field when it is not
  • Browser/native dropdown hidden when ours is open, shows when it is not
  • Tab selects current entered text and moves to next field
  • This behavior based on w3c pattern and this accessible combobox example
• Abstracts the field using this combobox into a sub-element.
• Adds validations to existing validations on the value field.
  • Note: There is a bug where the text-area field is not highlighted when it is invalid. (Followup bug report in Gitlab UI ▶ gitlab-ui#715 (closed))
• Adds related tests.

Adds documentation in related issue ▶ !29325 (merged).

Should we really be encouraging users to enter their credentials like this?

See #26777 (comment 305798396) for discussion and resolution of this point

Screengifs

Still screens

dropdown open	item selected (arrow down)
valid input (validated)	invalid input (value error)
invalid input (mask error)	invalid input (both errors)
valid input (no validation function)	native dropdown without input

Test locally

1. Be sure to enable :new_variables_ui and :ci_key_autocomplete feature flags on your local environment
2. Go to :user/:project/-/settings/ci_cd
3. Click Expand button for Variables section
4. Click Add Variable button
5. Now you got the popup! Woo!

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required) >> !29325 (merged).
• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

See #26777 (comment 305798396) for discussion and resolution of security implications