Add getVDBType() method to the VDB interface

Because Teiid supports mutliple VDB types, any tooling designed to interact (edit, deploy, undeploy, display etc...) with the runtime VDBs will need to be able to easilly distinguish what each VDB type is.

Currently Teiid Designer utilizes PREVIEW and defeault VDBs with vdb file extensions and there are 2 types of dynamic VDBs, xml and zip files backed by DDL model metadata definitions.

The current VDB interface currently doesn't provide a type.

Request adding something like:

    public enum VDBType {DEFAULT, PREVIEW, XML, DDL}

    /**
     * @return the vdb type
     */
    public VDBType getVDBType();