Enhance experience when multiple authentication methods are available

*This issue was originally created by [**@pmaier1**](https://github.com/pmaier1) at .*

Currently, the experience is good enough when:

- there is only one IDP and no password auth, because the upstream IDP login happens right away
- there is only password auth, because the user is presented with a username/login form

We need to enhance the experience when there are multiple upstream IdPs, and/or a mix of upstream IdPs and local password authentication options.

```[tasklist]
### Tasks
- [x] #2065
- [ ] https://github.com/element-hq/matrix-authentication-service/issues/3244
- [ ] Users can discover which IdP they used from their username (?)
```