constants: add GAME_TICK_LENGTH and use where needed

Also update many usages of 20ms to CLIENT_TICK_LENGTH

runelite-api/src/main/java/net/runelite/api/Constants.java

@@ -89,4 +89,12 @@ public class Constants
 	 * the maximum framerate of 50 fps.
 	 */
 	public static final int CLIENT_TICK_LENGTH = 20;
+
+	/**
+	 * The number of milliseconds in a server game tick.
+	 * <p>
+	 * This is the length of a single game cycle under ideal conditions.
+	 * All game-play actions operate within multiples of this duration.
+	 */
+	public static final int GAME_TICK_LENGTH = 600;
 }