Compat login : ability to use a REST provider as a password provider

[matrixbot]

This issue was originally created by @raphaelbadawi at matrix-org/matrix-authentication-service#3168.

Hello,

I'm working to make (soon) a PR on this.

Currently MAS supports local (DB) passwords compat login. Passwords provided by REST providers (e. g. MA1SD) are not supported yet. I'm working to make this configurable so if a REST password provider is filled in config.yaml, then compat login will happen against the REST password provider.

[raphaelbadawi]

PR here => #3193