[Bleeding] Add ProjectileSource interface. Addresses BUKKIT-1038, BUKKIT-1156

By: t00thpick1 <t00thpick1dirko@gmail.com>

paper-api/src/main/java/org/bukkit/projectiles/BlockProjectileSource.java

@@ -0,0 +1,13 @@
+package org.bukkit.projectiles;
+
+import org.bukkit.block.Block;
+
+public interface BlockProjectileSource extends ProjectileSource {
+
+    /**
+     * Gets the block this projectile source belongs to.
+     *
+     * @return Block for the projectile source
+     */
+    public Block getBlock();
+}

paper-api/src/main/java/org/bukkit/projectiles/ProjectileSource.java

@@ -0,0 +1,28 @@
+package org.bukkit.projectiles;
+
+import org.bukkit.entity.Projectile;
+import org.bukkit.util.Vector;
+
+/**
+ * Represents a valid source of a projectile.
+ */
+public interface ProjectileSource {
+
+    /**
+     * Launches a {@link Projectile} from the ProjectileSource.
+     *
+     * @param projectile class of the projectile to launch
+     * @return the launched projectile
+     */
+    public <T extends Projectile> T launchProjectile(Class<? extends T> projectile);
+
+    /**
+     * Launches a {@link Projectile} from the ProjectileSource with an
+     * initial velocity.
+     *
+     * @param projectile class of the projectile to launch
+     * @param velocity the velocity with which to launch
+     * @return the launched projectile
+     */
+    public <T extends Projectile> T launchProjectile(Class<? extends T> projectile, Vector velocity);
+}