Allow to define a Nuxeo Schema from a element of an XSD schema

When using XSD Schema that are generated by tools, a user may end up having a schema that has only 1 field.
This is not really efficient nor nice.

We can not easily have an automatic detection for these kind of case since this may introduce compatibility issues.

However, we could extend the existing Extension Point to be able to explictly ask for that.

<extension target="org.nuxeo.ecm.core.schema.TypeService" point="schema">
    <schema name="sample" xsdRootElement="sampleType" src="OSGI-INF/schema12.xsd" prefix="st"/>
</extension>