Relay open directives from Jakarta Persistence to implementations

Spec implementers need to access the persistent attribute values (and/or maybe other things). Under the JPMS system, this usually requires users to add corresponding `opens` directives to their module info:
```java
opens my.package.with.entities to some.jakarta.persistence.implementation.provider.module;
```

Changing providers(implementations) also requires changing these open directives. The suggestion is to let users declare the directives for the spec module and spec module can then relay them to the implementor:

```java
opens my.package.with.entities to jakarta.persistence;
```

An implementation idea of this for Jakarta Validation:
* https://github.com/jakartaee/validation/pull/340

Start by reading the issue proposal and the linked Jakarta Validation pull request #340, then compare how that project handles JPMS opens directives. Define the persistence specification and implementation behavior needed for users to target jakarta.persistence, including how completion would be verified.

Written by the indexing model from the issue text.