Add prevent-proxy-connections option to make sending client IP to Mojang toggleable

api/src/main/java/com/velocitypowered/api/proxy/config/ProxyConfig.java

@@ -62,6 +62,15 @@ public interface ProxyConfig {
    */
   boolean isOnlineMode();
 
+  /**
+   * If client's ISP/AS sent from this proxy is different from the one from Mojang's
+   * authentication server, the player is kicked. This disallows some VPN and proxy
+   * connections but is a weak form of protection.
+   *
+   * @return whether to prevent client proxy connections by checking the IP with Mojang servers
+   */
+  boolean shouldPreventClientProxyConnections();
+
   /**
    * Get a Map of all servers registered in <code>velocity.toml</code>. This method does
    * <strong>not</strong> return all the servers currently in memory, although in most cases it