RequestServer+LibProtocol: Add an 'EnsureConnection' IPC endpoint

This will allow LibWeb (and other components) to request a connection to
be premade and cached, to make subsequent loads faster.

Userland/Services/RequestServer/ConnectionCache.h

@@ -13,6 +13,15 @@
 #include <LibCore/Timer.h>
 #include <LibTLS/TLSv12.h>
 
+namespace RequestServer {
+
+enum class CacheLevel {
+    ResolveOnly,
+    CreateConnection,
+};
+
+}
+
 namespace RequestServer::ConnectionCache {
 
 template<typename Socket>