Update support of MSC2965 OAuth 2.0 Authorization Server Metadata discovery

[sandhose]

MSC2965 was updated to use a C-S API endpoint to retrieve the authorisation server metadata, instead of relying on the OpenID Connect Discovery spec.

One thing that is a little annoying, is that the SDK currently relies on crates originally published by MAS, that aren't updated anymore, and that were pretty pedantic about the server metadata discovery.

This means that a lot of the logic might have to be backported to within the SDK instead, so even if the MSC2965 change is supposed to be relatively small, in practice it might be bigger to do for the SDK.

Note that this isn't too urgent, as MAS intends to keep the old endpoint working for some time, to avoid breaking other apps.

Synapse implementation: element-hq/synapse#18093