Support for JTA 2.1 read-only transaction management

[jhoeller]

Following up on #35633, the UserTransaction read-only semantics are still in discussion. If they turn out to be stricter than Spring's read-only hint, we should only apply them when configured with an explicit enforceReadOnly=true flag at the Spring JtaTransactionManager level (similar to the same-named flag in DataSourceTransactionManager).