ENH: Support @Index platforms attribute - platform specific indexes

This is a potential alternative to using extra-dll.xml. Might be
appropriate to use over extra-dll in small simple cases.

We might look to extend this in the future to allow full raw platform specific ddl - the ability to specify the entire create index statement.

src/main/java/io/ebeaninternal/server/deploy/IndexDefinition.java

@@ -1,5 +1,7 @@
 package io.ebeaninternal.server.deploy;
 
+import io.ebean.annotation.Platform;
+
 /**
  * Holds multiple column unique constraints defined for an entity.
  */
@@ -9,12 +11,18 @@ public class IndexDefinition {
 
   private final String name;
 
+  private final Platform[] platforms;
+
   private final boolean unique;
 
-  public IndexDefinition(String[] columns, String name, boolean unique) {
+  /**
+   * Create from Index annotation.
+   */
+  public IndexDefinition(String[] columns, String name, boolean unique, Platform[] platforms) {
     this.columns = columns;
     this.unique = unique;
     this.name = name;
+    this.platforms = platforms;
   }
 
   /**
@@ -24,6 +32,7 @@ public class IndexDefinition {
     this.columns = columns;
     this.unique = true;
     this.name = null;
+    this.platforms = null;
   }
 
   /**
@@ -47,4 +56,10 @@ public class IndexDefinition {
     return columns;
   }
 
+  /**
+   * Return the platforms this index applies to.
+   */
+  public Platform[] getPlatforms() {
+    return platforms;
+  }
 }