Added a warning prompt if the user tries to use an API cert instead of a CLI cert

Summary: Fixes T9692. Instead of disallowing API tokens entirely, we're going to just warn the user that they might not want to do that. After that, they can proceed if they want to.

Test Plan:
Run arc install-certificate.
Manually go to Settings → Conduit API Tokens in the web UI.
Generate an API token explicitly, which should have the form api-**.
Paste that into the prompt on the CLI.
It will give you a warning prompt then ask if you'd like to proceed anyway (defaults to No).

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T9692

Differential Revision: https://secure.phabricator.com/D16448