[13.x] feat: add chaperone support for BelongsToMany pivot models

[calebdw]

Hello!

Many times when using custom pivot models, the BelongsTo relations can be defined on the model---however, trying to use them requires loading the models from the database even though both of the models are available during the BelongsToMany hydration

This PR adds chaperone support to the BelongsToMany relation, if a custom pivot model is not provided then calling ->chaperone() is a no-op. Any relations that don't exist are skipped (some pivot models only define one relation or even no relations); however, if a relation name is explicitly provided and the corresponding relation does not exist then an exception is thrown

class PostTagPivot extends Pivot
{
    public function post(): BelongsTo
    {
        return $this->belongsTo(Post::class);
    }

    public function tag(): BelongsTo
    {
        return $this->belongsTo(Tag::class);
    }
}

class Tag extends Model
{
    public function posts(): BelongsToMany
    {
        return $this->belongsToMany(Post::class)
            ->using(PostTagPivot::class)
            // automatically guesses relationship names
            ->chaperone();
            // but can override if needed or non-standard
            ->chaperone(declaring: 'tag', related: 'post');
    }
}

class Post extends Model
{
    public function tags(): BelongsToMany
    {
        return $this->belongsToMany(Tag::class)
            ->using(PostTagPivot::class)
            ->chaperone(declaring: 'post', related: 'tag');
    }
}

Thanks!