feat: add one to one relationship for embedding

BREAKING CHANGE: For the cases where one to one relationships are
detected, json objects will be returned instead of json arrays of length
1.

If you wish to override this behavior, you can use computed
relationships to return arrays again.

test/spec/Feature/Query/RpcSpec.hs

@@ -269,6 +269,20 @@ spec actualPgVersion =
           ]|]
           { matchHeaders = [matchContentTypeJson] }
 
+      it "can embed an O2O relationship" $ do
+        get "/rpc/allcapitals?select=name,country(name)"
+          `shouldRespondWith` [json|[
+            {"name":"Kabul","country":{"name":"Afghanistan"}},
+            {"name":"Algiers","country":{"name":"Algeria"}}]
+          |]
+          { matchHeaders = [matchContentTypeJson] }
+        get "/rpc/allcountries?select=name,capital(name)"
+          `shouldRespondWith` [json|[
+            {"name":"Afghanistan","capital":{"name":"Kabul"}},
+            {"name":"Algeria","capital":{"name":"Algiers"}}
+          ]|]
+          { matchHeaders = [matchContentTypeJson] }
+
       when (actualPgVersion >= pgVersion110) $
         it "can embed if rpc returns domain of table type" $ do
           post "/rpc/getproject_domain?select=id,name,client:clients(id),tasks(id)"