Expose GetSchemaMapping through IFeatureService

[jng]

The GetSchemaMapping API is not exposed through any of Maestro's service APIs, so add the following API in IFeatureService

public interface IFeatureService
{
    ...
    ConfigurationDocument GetSchemaMapping(string provider, string partialConnectionString);
}

This wraps the GETSCHEMAMAPPING mapagent request for http and MgFeatureService.GetSchemaMapping for Local/LocalNative

This API allows us to construct default configuration documents instead of us having to do it "by hand" as we currently do, though we would only do this for the WMS feature source editor, as it give us a better base document to address issues like #2117, where such information would be lost constructing it manually as we currently do.

[jng]

Fixed trunk (r7125) and 4.0.x (r7130)