Add in custom inventory fields and/or template hooks

[kculmback]

I have a client who, when updating inventory levels for a given product/variant, would like to be able to note what the price per item was so their sales person can know what kind of deals they can make for customers. For example, 100 of Item X was purchased at $5 per item.

The notes field on inventory updates gets them pretty close, but as far as I'm aware, that information can only be viewed by going to /admin/commerce/inventory/levels/default and then selecting the variant you want to look at and clicking the History tab. This would work okay for their sales person, but they also like to have a spreadsheet with this information available. The PR here would definitely be helpful for that.

Ideally though there would be more flexibility for developers to add extra information to inventory level updates.