Export of product links

Use product_links module.

Create a `magento.product.link`.
Add an option which when activated, automatically creates the `magento.product.link` for all the backends when a new link is created.

Use the usual @on_record_create for creation.

Links do not have an ID on Magento, instead, we refer to a link with the IDs of the 2 products and the type of the link.

For modification, create a new event @on_product_link_write, it keeps the ids of the 2linked products when they are modified so the job can still refer to them in order to remove the modified link.
For unlink, create a new event as well, which keeps the 2 ids too in order to remove the link.