Fetch access token

This API allows userapp and sysapp to fetch access tokens. For userapp, use this API after the /authorize API and the subsequent callback. For userapp this endpoint supports generation of access token from authorization code or from refresh token, sent in the previous authorize endpoint's callback. For sysapp this API can directly be used to get an access token, no prior /authorize end point call is needed.
NOTE: Fetching a token for a sysapp via the browser on our developer portal WILL NOT WORK as browser requests to the token endpoint must use PKCE. Instead, send the request through a server side/native method and ensure the 'Origin' header is not present.