Support push down join optimizations in DataFrameReader when loading from JDBC

val ordersDF = spark.read
  .format("jdbc")
  .option("url", "jdbc:postgresql:dbserver")
  .option("dbtable", "northwind.orders")
  .option("user", "username")
  .option("password", "password")
  .load().toDS
  
val productDF = spark.read
  .format("jdbc")
  .option("url", "jdbc:postgresql:dbserver")
  .option("dbtable", "northwind.product")
  .option("user", "username")
  .option("password", "password")
  .load().toDS
  
ordersDF.createOrReplaceTempView("orders")
productDF.createOrReplaceTempView("product")

// Followed by a join between them:
val ordersByProduct = sql("SELECT p.name, SUM(o.qty) AS qty FROM orders AS o INNER JOIN product AS p ON o.product_id = p.product_id GROUP BY p.name")