Provide helpers for interactive registration

[jplatte]

Currently, ruma-client only has support for non-interactive registration, which is rather limiting. We should look into supporting interactive registration. Maybe matrix-rust-sdk already does this and we can copy what they're doing.

[DevinR528]

I'll just throw this link here, this is what I needed for registration to work in rumatui https://github.com/DevinR528/rumatui/blob/master/src/client/ruma_ext/auth.rs

What does interactive mean in this context? Quite the old issue so maybe this is irrelevant by now.

[zecakeh]

Registration in Matrix uses UIAA, which doesn't seem to be supported by ruma-client