Add {:unsafe_fragment, ...} support to RETURNING clause

This adds support for raw SQL fragments in the RETURNING clause,
matching the existing pattern used by conflict_target.

This enables use cases like returning computed expressions:
  RETURNING id, (price = EXCLUDED.price) AS was_skipped

Author: henryzhan013

lib/ecto/adapters/postgres/connection.ex

@@ -1204,6 +1204,9 @@ if Code.ensure_loaded?(Postgrex) do
     defp returning([]),
       do: []
 
+    defp returning({:unsafe_fragment, fragment}),
+      do: [" RETURNING ", fragment]
+
     defp returning(returning),
       do: [" RETURNING " | quote_names(returning)]
 

test/ecto/adapters/postgres_test.exs

@@ -1877,6 +1877,10 @@ defmodule Ecto.Adapters.PostgresTest do
 
     query = insert("prefix", "schema", [], [[]], {:raise, [], []}, [])
     assert query == ~s{INSERT INTO "prefix"."schema" VALUES (DEFAULT)}
+
+    # With unsafe_fragment in returning
+    query = insert(nil, "schema", [:x, :y], [[:x, :y]], {:raise, [], []}, {:unsafe_fragment, ~s{"id", ("x" = "y") AS "was_equal"}})
+    assert query == ~s{INSERT INTO "schema" ("x","y") VALUES ($1,$2) RETURNING "id", ("x" = "y") AS "was_equal"}
   end
 
   test "insert with on conflict" do