[Turbo] Fix broadcasting an entity whose identifier is made of associations - #3847

…ations

| Q              | A
| -------------- | ---
| Bug fix?       | yes
| New feature?   | no
| Deprecations?  | no
| Documentation? | no
| Issues         | Fix symfony#1856
| License        | MIT

Entities whose primary key is made up of associations (the usual join-entity shape) couldn't be broadcast at all, as reported in symfony#1856. `ClassMetadata::getIdentifierValues()` returns the related entities themselves instead of their identifiers, and every consumer then runs `implode('-', $id)` on them, which throws `Object of class App\Entity\ClassA could not be converted to string`.

`IdAccessor::getIdentifierValues()` now reads the identifier and replaces any field mapped as an association with the identifier of the entity it points to. It checks the mapping of the field, not the class of the value, so it still works when the association is a lazy proxy, and an identifier held in a value object such as a UUID is left alone since it's a plain column.

Both producers now go through this accessor: `IdAccessor::getEntityId()`, and `BroadcastListener`, which used to read the identifier values itself and never reached the accessor at all: that's the exact path in the reported stack trace. The listener still gets its `EntityManager` from the flush event it's already handed, so nothing new is injected into it. This is covered by tests on the create path and the update path; the latter reloads the entity first, which under Doctrine ORM 2 turns the identifier associations into real proxies.