Add last_purchased and total_purchased to product variants

[ryanmitchell]

This PR adds new data to product variants, which is updated when an order is placed.

It should allow for sorting based on popularity (total purchased) or date (last purchased).

The format it is stored in is as follows:

total_purchased: 
  - lifetime: 100
  - 2023:
     - total: 100
  	 - 10: 50
     - 09: 25
     - 08: 25

[edit] before this merges I need to add test coverage for this webhook as there currently is none.

[ryanmitchell]

Any thoughts on this @edalzell - would be interested to hear what you think.