Add an SSO login link to the user/login page

[balsama]

Motivation

Users are linked to the SSO login page from Acquia Cloud Hosting. Users who find themselves on the regular /user/login page after that may not have set a password and won't have a way to login without knowing the specific path to Acquia SSO.

Proposed changes

Add a "Sign in with Acquia ID" link to the /user/login page conditionally displayed only when the site is running on Acquia hosting (SSO won't work in other environments.

Alternatives considered

None

Testing steps

• Verify that the "Sign in with Acquia ID" button is present on the login page when site is running on Acquia hosting
• Verify that the "Sign in with Acquia ID" button logs in the user and redirects them to the dashboard
• Verify that the "Sign in with Acquia ID" button is not present in environments that are not Acquia hosting

[acquia-stalebot-platauto]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Please remove the stale label to avoid it being closed. Thank you for your contributions. More info: https://github.com/acquia/devops-github-administration/blob/main/docs/operations_related_to_repositories.md#acquia-stale-bot