Add configuration interface to expose certain config values (#12273)

paper-server/src/main/java/io/papermc/paper/configuration/PaperServerConfiguration.java

@@ -0,0 +1,9 @@
+package io.papermc.paper.configuration;
+
+public class PaperServerConfiguration implements ServerConfiguration {
+
+    @Override
+    public boolean isProxyOnlineMode() {
+        return GlobalConfiguration.get().proxies.isProxyOnlineMode();
+    }
+}