Add selling_plan_allocation to Web Pixels checkout.line_item object

Discussed the potential schema heavily in Slack. Essentially, we want to limit the amount of fields we add to avoid adding maintenance and avoid having to load unnecessary fields onto every single checkout. This schema includes the bare minimum merchants are requesting/need.
A handful of fields that are in the liquid schema could not be populated given the fields that are already being requested in the API calls. We don't want to add to these api calls because that would decrease performance / increase latency on checkout. This is summarized here.

Shopify / ui-extensions