Create type interface for callback URL

[jakeleventhal]

amazon-sp-api/README.md

Line 175 in f7db3be

If you want to exchange an authorization code of a seller you can use the `.exchange()` function of the client. The neccessary authorization code is returned to your callback URI as `spapi_oauth_code` when a seller authorizes your application ([see authorization workflow in docs](https://developer-docs.amazon.com/amazon-shipping/docs/authorizing-selling-partner-api-applications)) or via a call to the `getAuthorizationCode` operation if you want to authorize a seller for the SP-API who has previously authorized you for the MWS API (the `getAuthorizationCode` workflow is explained in the [Grantless operations](#grantless-operations) section).

There should be a type interface for the request query in the SP callback URL