feat(reflect): Expose Assembly metadata field (#542)

This enables tools that use `jsii-reflect` to consume and reason over
the values in the metadata field.

Author: RomainMuller

packages/jsii-reflect/lib/assembly.ts

@@ -36,6 +36,13 @@ export class Assembly {
     return this.spec.description;
   }
 
+  /**
+   * The metadata associated with the assembly, if any.
+   */
+  public get metadata(): { readonly [key: string]: any } | undefined {
+    return this.spec.metadata;
+  }
+
   /**
    * The url to the project homepage. Maps to "homepage" from package.json.
    */