Add Attribute and AttributeModifier API.

Thanks to __0x277F and Meeh on #spigot-dev for implementation advice.

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/attribute/AttributeInstance.java

@@ -0,0 +1,60 @@
+package org.bukkit.attribute;
+
+import java.util.Collection;
+
+/**
+ * Represents a mutable instance of an attribute and its associated modifiers
+ * and values.
+ */
+public interface AttributeInstance {
+
+    /**
+     * The attribute pertaining to this instance.
+     *
+     * @return the attribute
+     */
+    Attribute getAttribute();
+
+    /**
+     * Base value of this instance before modifiers are applied.
+     *
+     * @return base value
+     */
+    double getBaseValue();
+
+    /**
+     * Set the base value of this instance.
+     *
+     * @param value new base value
+     */
+    void setBaseValue(double value);
+
+    /**
+     * Get all modifiers present on this instance.
+     *
+     * @return
+     */
+    Collection<AttributeModifier> getModifiers();
+
+    /**
+     * Add a modifier to this instance.
+     *
+     * @param modifier to add
+     */
+    void addModifier(AttributeModifier modifier);
+
+    /**
+     * Remove a modifier from this instance.
+     *
+     * @param modifier to remove
+     */
+    void removeModifier(AttributeModifier modifier);
+
+    /**
+     * Get the value of this instance after all associated modifiers have been
+     * applied.
+     *
+     * @return the total attribute value
+     */
+    double getValue();
+}