Implement AggregationService

AggregationService is used to inform a client that some of the results
might be aggregated, and to allow setting up aggregation sources.
Today, this resource only contains the basic fields, as well as
"Enabled", which informs the client of the fact that the service is
aggregation enabled.

AggregationService was one of the schemas we ignored, so this adds it to
the supported list.

Tested: Redfish service validator passes.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ifb16a86ff81e387f01016a83f9e69240c8928614