EVG-7427: add user manager for service users

[Kimchelly]

JIRA: https://jira.mongodb.org/browse/EVG-7427

This is the first part of making evergreen support both service users who can use the API and regular users who use Okta. The second part will allow you to combine user managers. I'm assuming the set of service accounts we need is so small that it doesn't really matter if we manually populate the service account username, API key, and roles.

Another thing we could try for the rollout is enabling Okta, OpenDJ LDAP, and the service user manager so that the migration doesn't go as painfully.

• Add settings for API-only auth users (similar to naive user manager but can only access the API and can't log into the UI).
• Connect with BasicUserManager. They can't use login cookies, so things like GetUserByToken and CreateUserToken won't work, so they can't use the UI. However, getting the user by ID and verifying their API key will work, so they can access the API.