Previously, both Atomikos and Bitronix were bound on the `spring.jta`
namespace which makes very hard to figure out which property belong to
which implementation. Besides, `AtomikosProperties` only exposed public
setter which does not generate any useful meta-data.

This commit moves the external configuration for Atomikos and Bitronix to
`spring.jta.atomikos.properties` and `spring.jta.bitronix.properties`
respectively. It also improves the meta-data support for those two
namespaces.

Closes gh-5165