Amazon SP: Support (even) more API endpoints

[sherifnada]

Tell us about the problem you're trying to solve

Inventory
Shipments
Vendor Inventory
Vendor Shipments
FBA Inventory
FBA Orders
FBA Shipments

[htrueman]

Product Requirements & features

1. What is the “ideal end state” for the features offered by this connector? At a minimum mention:
   a. Output schema
   All schemas are documented with swagger.
   b. Authentication
   AWS users complicated auth system, which is documented here. Already implemented in 🚀 Amazon SP: expand endpoint support #4863.
   c. Include more topics as you see fit depending on the connector (etc…)
   We may use https://github.com/saleweaver/python-amazon-sp-api lib for some code samples and inspiration.
   It is also not clear for now on how to get list Shipments records, going to research later.
2. What is the release roadmap for this connector?
   a. Which features are absolutely necessary for the MVP release?
   First we are going to implement FBA Inventory (seems to be already implemented), FBA Orders, FBA Shipments streams. As they are records streams and easy to implement. Then Vendor Inventory, Vendor Shipments streams. Inventory and Shipments need to be researched more before implementation.
3. Check if and how Fivetran or Stitch Data support this connector
   Fivetran does not support Amazon SP connector.
   Same for Stitchdata.

Implementation Scoping

• Describe the pagination strategy used by the API
  It's NextToken query param in all the streams. If we add this token, then no other params are supported.
• Describe the authentication strategy used by the API. If Oauth, go into detail about how the token can be obtained and if there are any special gotchas.
  Oauth. Auth strategy is really complex. But shortly: we need to transform, encode and subscribe initial credentials, then get access/refresh token from Amazon, then use it to auth requests. More details in docs
• Describe how we will populate the sandbox account with data
  Now we are using our test account, but there are no actual data in most streams. Thus we need to either user some prod account (I can't use Amazon SP API (owned by cart.com) creds from LastPass, as there are no enough permissions) or fill test account with some real data.
• How will rate limiting be handled?
  Rate limits are specified in the docs for each API type https://github.com/amzn/selling-partner-api-docs/tree/main/references These may be configured and for now our default backoff approach should be enough handle it.
• Type of the API? (REST/SOAP/GraphQL/other)
  REST

Integration details
See guide from Amazon: https://github.com/amzn/selling-partner-api-docs/blob/main/guides/en-US/developer-guide/SellingPartnerApiDeveloperGuide.md
Google sheet connector details: https://docs.google.com/spreadsheets/d/1s-MAwI5d3eBlBOD8II_sZM7pw5FmZtAJsx1KJjVRFNU/edit#gid=1972105305