Make product creation from bom line expolde customizable.

[codingforfun]

Description of the issue/feature this PR addresses:
Make it easier to customize product dict creation during mrp.bom line explode.

Current behavior before PR:
Currently to customize it would be necessary to copy all the code of the function.

Desired behavior after PR is merged:
Override only _bom_explode_product() to modify the returned data.

Example: For a special customization in my own code I need the ID of the parent mrp.bom the product dict is comming from.
This simple change allows to hook in easily

I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

[codingforfun]

Upstream PR: odoo#13311

[pedrobaeza]

This was not accepted for 8.0 for us when we proposed it, so we created https://github.com/OCA/manufacture/tree/8.0/mrp_hook. Depends on that module and you will have it. On v9, the hook was accepted, so you can simply remove it in your migration path.