Add `retrieve_line_items` function for `CheckoutSession`

[belyakov-am]

Summary

Add missing method for checkout session

https://docs.stripe.com/api/checkout/sessions/line_items

Checklist

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

[arlyon]

CI approved, TY

[arlyon]

:tada: This PR is included in version 0.36.0 :tada:

The release is available on:

• v0.36.0
• GitHub release

Your semantic-release bot :package::rocket: