Explicitly declare to Quarkus that ORM XML mapping is required for the outbox extension

Hibernate ORM's XML mapping used to be enabled by default in Quarkus, but that was unintentional and caused performance issues.

It's going to be disabled by default in Quarkus 1.11 but an SPI will allow to re-enable it when some extensions really need it; see
https://github.com/quarkusio/quarkus/pull/13929.

I'm going to send a PR with the required changes soon, but it will be just a draft, since the version of Quarkus that introduces this SPI wasn't released yet.