Add missing route handler for legacy Apps

[Jeidnx]

As discussed in #286 there is a missing route handler for _matrix/client/r0/login/sso/redirect/[idp]. This is not in the current matrix spec, but simple enough to implement since its the same as the equivalent v3 endpoint. Current versions of at least Element Classic and Schildichat do not work without this endpoint.

I've written a very crude patch to implement this, but maybe someone who knows the codebase can do a proper fix.

route_fix.patch

[jevolk]

Thank you for the patch. I'm going to port it over to Ruma in a moment.

[CEbbinghaus]

@jevolk if I am not mistaking this patch is still not in main, I had a look and couldn't get it working.

[jevolk]

Sorry about that 😭 I did apply 60d4181 but I guess something was missed still here.

[jevolk]

The route was added here: matrix-construct/ruma@0130f6a and appears to work for me (now only /v1/ returns an M_UNRECOGNIZED).