hide sliding sync behind explicit option

We want to make sure users know this sliding sync impl is pretty buggy
before they attempt to use it.

src/api/well_known.rs

@@ -51,8 +51,14 @@ 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: Some(client::SlidingSyncProxyInfo {
-            url: base_url,
-        }),
+        sliding_sync_proxy: services()
+            .globals
+            .config
+            .server_discovery
+            .client
+            .advertise_sliding_sync
+            .then_some(client::SlidingSyncProxyInfo {
+                url: base_url,
+            }),
     })
 }

src/config.rs

@@ -96,6 +96,9 @@ pub(crate) struct ClientServerDiscovery {
     /// Controls whether HTTPS is used
     #[serde(default)]
     pub(crate) insecure: bool,
+
+    #[serde(default, rename = "advertise_buggy_sliding_sync")]
+    pub(crate) advertise_sliding_sync: bool,
 }
 
 #[derive(Debug, Deserialize)]