Add jdbc-vs-jpa module: Spring Data JDBC vs JPA on a shared Order aggregate

Same Customer/Order/OrderItem domain modelled with Hibernate/Spring Data JPA and
Spring Data JDBC side by side, both running through a shared StatementLoggingDataSource
so SQL-statement counts are directly comparable. 11 tests, 11 captured transcripts,
6 doc chapters. Companion repo for the ankurm.com article on when to drop the ORM.

jdbc-vs-jpa/docs/output/03-entity-graph-fixed-cost.txt

@@ -0,0 +1,10 @@
+Scenario: the same lookup, but through findWithItemsAndCustomerById (an
+@EntityGraph fetch), for an order with 1 item and an order with 5 items.
+
+statements for 1-item order:  1
+statements for 5-item order:  1
+
+Both are the same number — an entity graph turns the collection fetch into a
+single outer-join SELECT, so the statement count stops depending on item count.
+This is the fix for scenario 02, and it is also the shape Spring Data JDBC gives
+you by default with no annotation at all (see docs/05).