Directus as a SSO provider

Summary Add OIDC endpoints so that external services can use Directus directly as a single-sign on provider. Basic Example We would just need to implement these endpoints. Using existing libraries, it may be even simpler. Here's an example node-oidc-provider configuration: Motivation This would remove the need for external user management, like Keycloak, Ory, etc. And would allow for easier management of users directly with directus, adding user metadata, etc. I suggest it because if done right, it could be very low hanging fruit! Detailed Design The best place for this is likely an extension. IMPORTANT: This is only worthwhile if the Oauth2/OIDC implementation can be offloaded to external standardized libraries, where the added functionality is just a thin adapter around Directus's storage backend. The best maintained library seems to be this: Here is an example implementation: Directus Changes: * directus_oidc_clients: a place to store clien…