CONNECT engine JDBC to support JOIN clause push-down

This is a feature request to add JOIN clause push down for CONNECT engine JDBC.

Currently, if you issue a query to several CONNECT tables JOINed together that are all from the same remote database, and a WHERE clause is only specified for a single table, the other tables bring back all rows to do the join in MariaDB. This feature request is to "push down" the JOIN clause so that effectively the entire query is processed on the remote server and only the results are returned.

Example (all tables in the example below are CONNECT JDBC tables):

SELECT * 

FROM Customers

JOIN Orders ON (Orders.Customer_ID = Customers.Customer_ID)

JOIN Order_Lines ON (Order_Lines .Order_ID = Orders.Order_ID)

JOIN Items ON (Items.Item_ID = Order_Lines.Item_ID)

WHERE Customer_ID = 1234

Something like the above query is very common for us, where we are looking at data across several remote tables but only one of the tables has a restriction (all orders for a certain customer, or all orders for a certain date range).