SDO-55 - Create an EntityResolver like concept for resolving imported/includedschemas

Hello All,

As discussed last week, most SDO implementations have some means of
resolving System URIs.  There a couple of use cases for which this is
useful:

  
The user is loading there XML schema using a non-standard URI
(i.e. pointing to somewhere on there class path).

  
The user is specifying imported/included XML schemas using a
relative path.

In the EclipseLink SDO implementation we have introduced the concept of
a SchemaResolver.  This interface has the following signature:

public interface SchemaResolver {

    /**

     * @param parentXSD - The XSD doing the import or included (null
for root schemas), the system id can be obtained to resolve relative
URIs

     * @param namespace - The namespace portion of the import/include

     * @param systemID - The URI representing the location of the XML
schema.

     */

    public javax.xml.transform.Source
resolveSchema(javax.xml.transform.Source parentXSD, String namespace,
String systemID);

}

Java SE also has the concept of an LSResourceResolver
(http://java.sun.com/javase/6/docs/api/org/w3c/dom/ls/LSResourceResolver.html). 
This approach has the benefit of working well with the
javax.xml.validation APIs.

Hopefully we can get some discussion on this topic, in order to come up
with a solution that works for everyone.

-Blaise