Revert "Remove support for MSC3575 (sliding sync)"

This reverts commit d87848b9a6.

src/api/well_known.rs

@@ -37,5 +37,14 @@ pub(crate) async fn client(_: Ar<client::Request>) -> Ra<client::Response> {
     Ra(client::Response {
         homeserver: client::HomeserverInfo::new(base_url.clone()),
         identity_server: None,
+        sliding_sync_proxy: services()
+            .globals
+            .config
+            .server_discovery
+            .client
+            .advertise_sliding_sync
+            .then_some(client::SlidingSyncProxyInfo {
+                url: base_url,
+            }),
     })
 }