Allow POST on aggregation sources

It would be useful for both testing and production if one could create
aggregation sources dynamically, rather than using detected hardware.
Redfish sources might exist outside of the physical chassis in a way
that's detectable, so giving DC software a way to set up a non-trivial
aggregation topology make this more extensible.

Note, that as documented in AGGREGATION.md, only a single satellite
is supported by this feature.  This patch does not change that
behavior, and implementing an entity-manager satellite will override
a POST created AggregationSource.

Tested:

Example commands succeed, and return the expected results.
```
curl -vvvv -k --http1.1 --user "root:0penBmc" --request DELETE https://192.168.7.2/redfish/v1/AggregationService/AggregationSources/
curl -vvvv -k --http1.1 --user "root:0penBmc" https://192.168.7.2/redfish/v1/AggregationService/AggregationSources -H "Content-Type: application/json" --request POST --data '{"HostName":"https://localhost:8000"}'
```

Redfish service validator passes.

Change-Id: I15711c3075645291b5f2555eefb80bb80418e7e5
Signed-off-by: Ed Tanous <ed@tanous.net>
diff --git a/redfish-core/include/query.hpp b/redfish-core/include/query.hpp
index d38da62..a810198 100644
--- a/redfish-core/include/query.hpp
+++ b/redfish-core/include/query.hpp
@@ -167,7 +167,7 @@
     if constexpr (BMCWEB_REDFISH_AGGREGATION)
     {
         needToCallHandlers =
-            RedfishAggregator::beginAggregation(req, asyncResp) ==
+            RedfishAggregator::getInstance().beginAggregation(req, asyncResp) ==
             Result::LocalHandle;
 
         // If the request should be forwarded to a satellite BMC then we don't