Supporting OAuth device PIN flow. Getting code from oauth provider and show in output prompting merchant user to navigate to browser and authorize PIN.
The oauth client tries periodically, till the code expires, polling for access token in exchange for the device PIN. Once the merchant authorizes the PIN, the program can get the access token and proceed calling the API.
The program uses the access token in its HTTP Authorization headers as Bearer token to get access to Transaction/Terminal API
Moving oauth and api urls to constants defs