Support `com.devture.shared_secret_auth` for shared secret auth

[samip5]

It seems that currently while primary login is via SSO, it's not really possible to enable double puppeting.

Please support it if password auth is not enabled.

[sumnerevans]

https://github.com/mautrix/signal/blob/master/mautrix_signal/example-config.yaml#L154-L162

Note that you have to set m_login_password_support_enabled to true in the auth provider configuration for it to work.

[tulir]

This is for the non-m.login.password method, which is a valid feature request, although not sure if it'll be implemented