Public API: set line descriptions on quote create, and read and manage product add-ons

Two Public API gaps mean quotes and catalogs built from an external system still need manual work in Salesbuildr afterwards:

1. Line descriptions on quote create. When a quote is created through /public-api/quote, every line takes its name and descriptions from the catalog product. The line payload only accepts price fields (originalPrice, cost, price, markup, term), so quote-specific line text has to be edited by hand in the app.

2. Product add-ons. The product endpoints don't show or change a product's add-ons. You can't see which add-ons are linked, link or unlink one, or set its type (required or optional) and the "Add to quote" flag. With hundreds of products whose add-on mappings change regularly, keeping them right by hand isn't feasible.

What would help:

• Optional per-line name and description fields on quote create that override the catalog text for that line only.

• Add-ons on the product responses, plus a way to add, update and remove a product's add-ons (including "Add to quote") on the single and batch product endpoints, by id or external identifier.

This post combines two earlier requests: "Public API: set a custom description per quote line when creating a quote" and "Public API: Read and manage product add-ons, including the 'Add to quote' flag". Their votes and comments are merged in here.