Support login methods other than username/password (User-Interactive Authentication API) #157

Like Single Sign-On (SSO), and for stuff like SSO-only homeservers. Probably by implementing the User-Interactive Authentication API

This may become more relevant in the future where logging into Matrix is not username/password and is instead OIDC (or some other SSO), making synadm matrix login non-functional because it is only username/password.

Resources:

• https://github.com/element-hq/matrix-authentication-service
• https://areweoidcyet.com/

Considerations when implementing:

• synadm could be running on the admin's laptop, or on the server with a running Synapse (where a web browser is not available).
• OIDC: https://areweoidcyet.com/#next-gen-auth-aware-clients
• OIDC: https://areweoidcyet.com/#faqs

Note: I don't know if the OIDC thing is SSO. If that's not the case, this issue may have to split into SSO and future OIDC logins