Be able to test `/login/sso/redirect` in Complement
Spawning from
https://github.com/element-hq/sbg/pull/421#discussion_r1854926218 where
we have a proxy that intercepts responses to
`/_matrix/client/v3/login/sso/redirect(/{idpId})` in order to upgrade
them to use OAuth 2.0 Pushed Authorization Requests (PAR). We have some
Complement tests in that codebase that go over this flow and these
changes are required [in order for the URL's to line
up](d648c8ce3f/synapse/rest/client/login.py (L652-L673)).
