Port module to the new interface

Synapse introduced new generic modules system in v1.37.0. For more details see:
- https://element-hq.github.io/synapse/latest/upgrade.html#upgrading-to-v1390
- https://element-hq.github.io/synapse/latest/modules/porting_legacy_module.html
- https://element-hq.github.io/synapse/latest/modules/password_auth_provider_callbacks.html

This commit only updates the code needed for login. Once it is tested
and found working, the code needed to register users which used to work
with the legacy module api also needs to be updated.