[Feature Request] Redirect to auth provider immediately

I propose a new (optional) `auth` config option to configure docspell to redirect directly to the auth provider when the user is not logged in, e.g.

```
auth {
default-provider="keycloak"
}
```
Immediatly redirects the user to the auth configured under `openid` with the display name "keycloak". Leaving this value empty or on "local" would display the current login screen and the buttons underneath.