Fix `CheckoutSession::retrieve_line_items` parameters and associated stuct visibility

[nightspc]

Summary

Fixes calling the newly added CheckoutSession::retrieve_line_items function added in #541 by:

• publicly exporting the checkout_session_ext module
• changing the id type of the parameters to the parameters to CheckoutSession::retrieve_line_items function

Checklist

• [x] ran cargo make fmt
• [x] using conventional commits to hightlight user-facing fixes and features