[5.7] Added chunkById support in relations.

[dkulyk]

Pivot data doesn't attached to model If you use Builder::chunkById.

This PR is fixing this and using actual of relation keys for chunking.

Closes #26816.
Related to https://github.com/laravel/nova/issues/152

For 5.6 need separate PR?

[staudenmeir]

I'm wondering about the BelongsToMany implementation:
Should we also use $this->getRelated()->[...] for $column and $alias (as HasManyThrough does)?

In a User ↔ Role relationship:

• Your implemenation: role_user.role_id
• My suggestion: roles.id

It would be more consistent and in basically every case both columns contain the same value. The only exception would be a relationship that doesn't join the related model's primary key (e.g. roles.slug). But I'm not even sure what the expected/correct behavior would be in a case like this.