Log Agroal 'Connection acquired without transaction.' warning with stack trace right away

[famod]

Rationale: #18946 (comment)

Before:

2021-11-30 22:36:36,985 WARN  [io.agr.pool] (main) Datasource '<default>': Connection acquired without transaction.
2021-11-30 22:36:36,985 DEBUG [io.agr.pool] (main) Cause: : java.sql.SQLException: Connection acquired without transaction.
        at io.agroal.pool.ConnectionPool.afterAcquire(ConnectionPool.java:351)
        at io.agroal.pool.ConnectionPool.getConnection(ConnectionPool.java:245)
        at io.agroal.pool.DataSource.getConnection(DataSource.java:86)
        [...]

After:

2021-12-01 00:36:30,688 WARN  [io.agr.pool] (main) Datasource '<default>': java.sql.SQLException: Connection acquired without transaction.
        at io.agroal.pool.ConnectionPool.afterAcquire(ConnectionPool.java:351)
        at io.agroal.pool.ConnectionPool.getConnection(ConnectionPool.java:245)
        at io.agroal.pool.DataSource.getConnection(DataSource.java:86)
        [...]

The exception message check is far from pretty, but I don't see another way due to the lack of a dedicated listener method.
It would be nice if such a method could be added to a future Agroral release, /cc @barreiro.

[Sanne]

looks nice, thanks @famod .