SPI to have other extensions contribute (static) Hibernate Persistence Unit definitions

[yrodiere]

Description

#54493 occurs because quarkus-langchain4j has to rely on internal code to programmatically trigger the creation of persistence units.

We should offer a proper SPI for that instead.

Possibly related: #51198

Implementation ideas

No response

[quarkus-bot]

/cc @gsmet (hibernate-orm)

[mcruzdev]

Do you mind to get this one @yrodiere? If not I would like to take care

[yrodiere]

If you're interested, please go ahead!

The requirements are unfortunately not that precise, but broadly speaking:

1. The core of the problem is that we need a build item that says "persistence unit X must exist, and here is some additional config". Even though there is no mention of that persistence unit in user-provided configuration.
2. When it comes to that additional config, we need to allow customization of anything @beikov is trying to do there: https://github.com/quarkiverse/quarkus-langchain4j/pull/2189/changes#diff-9a1e3b653ddb72441dcac67f3e4637de2b14f4ce2d3bef39ad0152c3f2389944R117
3. ... but I'd recommend not going much beyond that. We can always add more later if necessary, but changing what we do expose will involve headaches related to breaking changes.
4. We can reuse some existing bits if necessary; e.g. AdditionalJpaModelBuildItem allows adding entities to the model and specifying which
5. When it comes to testing... I'm not sure, but I think you'll need to define an extension just to test that? I think there are a few examples of how to do that in integration-tests/test-extension. Maybe you can define a extension-that-customizes-hibernate-orm directory there, add a processor that produces the relevant build items in deployment/src/main, and add tests to deployment/src/test?