REST-API: Support reusing authentication from configured auth provider

Summary Directus supports external authentication providers. The REST-API from Directus requires a login via authentication flow (same flow which is used for login in Admin Panel). Clients which have not the option to use a web-frontend for authentication or which already have a valid session from the external authentication provider needs right now a separate session from Directus to access the REST-API. It would be helpful when the access token from the user session (which comes from the external IdP) could be used as Bearer token in Authorization header to accessing the REST-API from Directus. Basic Example Having a client application which already have an active and valid user session, e.g. from Keycloak. This application should now send data to Directus (which will be used as data hub in this constellation) and request data which the user can access to with the current user session. Motivation Since Directus is a truly headless CMS, it would be helpful …