feat: add count(distinct) as countdistinct aggregate

Added count(distinct) as
  countdistinct aggregate. Tested to work, and that it is gated when aggregates are off. Tested against collisions with
  tables and columns named countdistinct. Returns type based on Postgres rules for count(). Extended test coverage to paths including spread path.

Author: datavaultbuilder

test/spec/Feature/Query/AggregateFunctionsSpec.hs

@@ -224,6 +224,10 @@ allowed =
           get "/budget_expenses?select=...budget_categories(total_budget:budget_amount.sum())" `shouldRespondWith`
             [json|[{"total_budget": 9501.06}]|]
             { matchHeaders = [matchContentTypeJson] }
+        it "auto-aliases countdistinct() to 'count' inside a to-one spread" $ do
+          get "/budget_expenses?select=...budget_categories(budget_amount.countdistinct())" `shouldRespondWith`
+            [json|[{"count": 4}]|]
+            { matchHeaders = [matchContentTypeJson] }
         it "supports aggregates from a spread relationships grouped by spreaded fields from other relationships" $ do
           get "/processes?select=...process_costs(cost.sum()),...process_categories(name)&order=process_categories(name)" `shouldRespondWith`
             [json|[