Support iterable type for `HasMany`

Just a scratch:

```php
interface HasIterableType
{
    public TypeReflector $iterableType { get; }
}

class HasMany implements HasIterableType {}

public function getIterableType(): ?TypeReflector
{
    if ($this->hasAttribute(HasIterableType::class)) {
        return $this->getAttribute(HasIterableType::class)->iterableType;
    }

    // Fallback to docblock parsing
}
```