Postgres/MySQL/MSSQL: Fix region annotations not displayed correctly (#38936)

Fix region annotations not displayed correctly when returning timeend column 
as epoch timestamp and by that making sure that the returned data frame field 
named timeend is treated as time type.

Fixes #38533

pkg/tsdb/postgres/postgres_test.go

@@ -1199,6 +1199,32 @@ func TestPostgres(t *testing.T) {
 			// Should be in time.Time
 			assert.Nil(t, frames[0].Fields[0].At(0))
 		})
+
+		t.Run("When doing an annotation query with a time and timeend column should return two fields of type time", func(t *testing.T) {
+			query := &backend.QueryDataRequest{
+				Queries: []backend.DataQuery{
+					{
+						JSON: []byte(`{
+							"rawSql": "SELECT 1631053772276 as time, 1631054012276 as timeend, '' as text, '' as tags",
+							"format": "table"
+						}`),
+						RefID: "A",
+					},
+				},
+			}
+
+			resp, err := exe.QueryData(context.Background(), query)
+			require.NoError(t, err)
+			queryResult := resp.Responses["A"]
+			require.NoError(t, queryResult.Error)
+
+			frames := queryResult.Frames
+			require.Equal(t, 1, len(frames))
+			require.Equal(t, 4, len(frames[0].Fields))
+
+			require.Equal(t, data.FieldTypeNullableTime, frames[0].Fields[0].Type())
+			require.Equal(t, data.FieldTypeNullableTime, frames[0].Fields[1].Type())
+		})
 	})
 }