Custom Collections through delegation

Users regularly want to introduce their own collections. We don't want to expose the Collection interface due to its complexity there is too much risk. MongoDB ODM has a code generated solution for this, for ORM I propose we do this with delegation.

1. doctrine/collections introduces a DelegateCollection with all methods implemented as final, delegating to $delegatee, and one custom method getInternalDelgatee.
2. In ORM association configuration, allow optional "collectionType" information. When present, ORM converts with logic:
   • create collection new $assoc['collectionType](new PersistentCollection(...)) everywhere a PersistentCollection is created.
   • introspect collection $col = $col instanceof DelegateeCollection ? $col->getDelegatee() : $col
3. Improve AttributeDriver to detect that a collection type instanceof DelegateeCollection and set collectionType key automatically.
4. Precondition: In ORM, if collection types are configured and Composer\PackageVersions for doctrine/collection is not high enough, throw.

Example code:

class CommentCollection extends DelegateCollection
{
	public function getEnabled() {
		return $this->delegatee->filter(fn ($comment) => $comment->isEnabled);
        }
}

class Post
{
        private CommentCollection $comments;

	public function getComments(): CommentCollection
        {
             return $this->comments;
         }
}

Reference:

• MongoDB docs: https://www.doctrine-project.org/projects/doctrine-mongodb-odm/en/2.9/reference/custom-collections.html