feat: property support for max clientside channels (PaperMC#1557)

Author: SpigotRCE

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ConnectedPlayer.java

@@ -146,7 +146,7 @@
 public class ConnectedPlayer implements MinecraftConnectionAssociation, Player, KeyIdentifiable,
     VelocityInboundConnection {
 
-  public static final int MAX_CLIENTSIDE_PLUGIN_CHANNELS = 1024;
+  public static final int MAX_CLIENTSIDE_PLUGIN_CHANNELS = Integer.getInteger("velocity.max-clientside-plugin-channels", 1024);
   private static final PlainTextComponentSerializer PASS_THRU_TRANSLATE =
       PlainTextComponentSerializer.builder().flattener(TranslatableMapper.FLATTENER).build();
   static final PermissionProvider DEFAULT_PERMISSIONS = s -> PermissionFunction.ALWAYS_UNDEFINED;