Improve transaction management for @Sql scripts

Prior to this commit, the support for SQL script execution via @Sql
provided an algorithm for looking up a required
PlatformTransactionManager to use to drive transactions. However, a
transaction manager is not actually required for all testing scenarios.

This commit improves the transaction management support for @Sql so
that SQL scripts can be executed without a transaction if a transaction
manger is not present in the ApplicationContext. The updated algorithm
now supports the following use cases.

 - If a transaction manager and data source are both present (i.e.,
   explicitly specified via the transactionManager and dataSource
   attributes of @SqlConfig or implicitly discovered in the
   ApplicationContext based on conventions), both will be used.

 - If a transaction manager is not explicitly specified and not
   implicitly discovered based on conventions, SQL scripts will be
   executed without a transaction but requiring the presence of a data
   source. If a data source is not present, an exception will be thrown.

 - If a data source is not explicitly specified and not implicitly
   discovered based on conventions, an attempt will be made to retrieve
   it by using reflection to invoke a public method named
   getDataSource() on the transaction manager. If this attempt fails,
   an exception will be thrown.

 - If a data source can be retrieved from the resolved transaction
   manager using reflection, an exception will be thrown if the
   resolved data source is not the data source associated with the
   resolved transaction manager. This helps to avoid possibly
   unintended configuration errors.

 - If @SqlConfig.transactionMode is set to ISOLATED, an exception will
   be thrown if a transaction manager is not present.

Issue: SPR-11911