Implement furnace cook speed multiplier API. (#1437)

Adds methods `Furnace#getCookSpeedMultiplier()` and
`Furnace#setCookSpeedMultiplier(double)`.

This PR is basically 3516ae34ef for 1.13.

A test plugin may be found [here](https://gist.github.com/supertassu/fade0cce946261732c6299e1ec89290e).

Spigot-API-Patches/0152-Implement-furnace-cook-speed-multiplier-API.patch

@@ -0,0 +1,41 @@
+From 22461a04950e5ef28f7cd61b5d28b091759234d0 Mon Sep 17 00:00:00 2001
+From: Tassu <git@tassu.me>
+Date: Thu, 13 Sep 2018 08:45:01 +0300
+Subject: [PATCH] Implement furnace cook speed multiplier API
+
+Signed-off-by: Tassu <git@tassu.me>
+
+diff --git a/src/main/java/org/bukkit/block/Furnace.java b/src/main/java/org/bukkit/block/Furnace.java
+index ee8a95601..437c7c4fd 100644
+--- a/src/main/java/org/bukkit/block/Furnace.java
++++ b/src/main/java/org/bukkit/block/Furnace.java
+@@ -36,6 +36,26 @@ public interface Furnace extends Container, Nameable {
+      */
+     public void setCookTime(short cookTime);
+ 
++    // Paper start
++    /**
++     * Gets the cook speed multiplier that this {@link Furnace} will cook
++     * compared to vanilla.
++     *
++     * @return the multiplier, a value between 0 and 200
++     */
++    public double getCookSpeedMultiplier();
++
++    /**
++     * Sets the speed multiplier that this {@link Furnace} will cook
++     * compared to vanilla.
++     *
++     * @param multiplier the multiplier to set, a value between 0 and 200
++     * @throws IllegalArgumentException if value is less than 0
++     * @throws IllegalArgumentException if value is more than 200
++     */
++    public void setCookSpeedMultiplier(double multiplier);
++    // Paper end
++
+     @Override
+     public FurnaceInventory getInventory();
+ 
+-- 
+2.19.0
+