Marketplace: Endpoint for creating a billing_intent

[cpapazoglou]

A billing intent is the intention of a customer to go through with a transaction. The billing intent is created by a 3pd vendor after a customer selects their plan in the 3pd interfaces. Vendor should have a way to create [POST] this billing intent through an API.

• [ ] the endpoint accepts uuid (combination of user and site id), store_product_slug, price in usd, billing_interval ( monthly / annually ), return_url
• [ ] the endpoint returns a WP.com checkout url https://wordpress.com/checkout/[DOMAIN] or an error

A basic data structure for billing intents has been created in https://github.com/Automattic/wp-calypso/issues/67260

[gavande1]

WIP Diff created: D87610-code

[gavande1]

Diff separated for easy review- D88280-code

[gavande1]

Deployed part 2 in r253253-wpcom

[gavande1]

Diff created for tests: D88989-code

[cpapazoglou]

Since we are not doing anything catastrophic in production, I guess we can merge this and iterate.

[gavande1]

Deployed r253805-wpcom

[gavande1]

Test deployed r254043-wpcom