Support subsetting returned metadata

Add a new preference, metadata-scope, that can be specified on a metadata request. The value follows the syntax for the fragment portion of a context URL to describe the shape of results for which the client needs metadata. Clients can then extract the fragment portion of the context url to use in the preference to requests only the metadata required to describe that particular response. If the response spans multiple namespaces the service MAY include multiple schema elements in the response, one per namespace containing types used in the response. Since this is a preference, services can also ignore and just return the metadata with references to related metadata. Services that support the metadata-scope preference SHOULD include a preference-applied header specifying the contextUrl fragment identifying the results described by the returned metadata. Example: to include the metadata required to describe customers and their orders, the client could specify: http://sampleservice.com/$metadata Prefer: metadata-scope=Customers(Orders) Services can return preference-applied to specify the extent of the metadata included. For any included namespaces, the service must return at least the requested data, but data in related namespaces can be referenced through Include.