add expand to product endpoints

[cball]

This adds expand to products in the same way its being used throughout the rest of the library.

While it would be nice to be able to do this without a breaking change, I think it's more important to keep things consistent in the approach. This means that anyone using these product endpoints will have to update their code to pass nil for expand.

Let me know if anything additional is needed here.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@3c2d6ea). Learn more about missing BASE report.

Additional details and impacted files

@@          Coverage Diff           @@
##             main    #260   +/-   ##
======================================
  Coverage        ?   2.15%           
======================================
  Files           ?     194           
  Lines           ?    8864           
  Branches        ?       0           
======================================
  Hits            ?     191           
  Misses          ?    8673           
  Partials        ?       0           

Files	Coverage Δ
...es/StripeKit/Products/Products/ProductRoutes.swift	0.00% <0.00%> (ø)