Add Exchange the token code for a business token

[paulomcnally]

Exchange the token code for a business token
https://developers.facebook.com/docs/whatsapp/embedded-signup/onboarding-customers-as-a-solution-partner

Use the GET /oauth/access_token endpoint to exchange the token code returned by Embedded Signup for a business integration system user access token ("business token").

[piscespieces]

Hey @paulomcnally

Are you planning to work on this issue? — I literally implemented the token exchange with Net HTTP in one of my apps and it's very verbose! This would definitely be a great addition.