Config: Refresh Token Request with Custom Parameters

[zs-codex]

We'd like to be able to add custom parameters to the Refresh Token Request provided by the configuration client.

Specifically, we need a way to add the redirect_uri parameter to the refresh request:

curl -X POST \
  https://api.dexcom.com/v2/oauth2/token \
  -H 'cache-control: no-cache' \
  -H 'content-type: application/x-www-form-urlencoded' \
  -d 'client_secret={your_client_secret}&client_id={your_client_id}&refresh_token={your_refresh_token}&grant_type=refresh_token&redirect_uri={your_redirect_uri}'

The auth server returns an error if redirect_uri is missing. This is required by Dexcom but is general enough to support many other use cases.

[forgedbyte]

This is also required by recent Gitlab versions.

[andig]

The token refresh is handled by the RefreshTokenSource. How would an API look like that is able to supply additional headers? Are these the same headers as during initial token retrieval? If yes this might potentially be done using a http.Transport.