[Feature]: XEP-0493: OAuth Client Login

### Precondition - [x] I understood that this template is only for feature requests and not for bug reports ### Cross-checked related issues? - [x] I have cross-checked this overview https://github.com/monal-im/Monal/issues/322 as well as filtered for related labels https://github.com/monal-im/Monal/labels ### Describe your feature Hello, I'd like to set up an XMPP server for my hackerspace. We use Authentik as our Identity Provider. It looks like XEP-0493 currently isn't supported. As the hackspace's migration to Single Sign-On took a really long time, having OAuth/OIDC is a requirement. Currently there does not seem to be any client, though, that supports it. Having this would be great, as it'd make it easy for us, and other organisations, to host XMPP. :)