Refresh token automatically if token is expired

[lmammino]

This requires the following information:

• grant_type: refresh_token
• refresh_token
• client_id
• client_secret
• scope: openid
• audience: (same value as the client_id)

Teoretically a user could refresh a token by themselves by performing the following request:

curl -v -d 'grant_type=refresh_token&refresh_token=XXXX&client_id=XXX&client_secret=XXX&scope=openid&audience=XXX' -H 'Content-Type: application/x-www-form-urlencoded' 'https://<cognito_user_pool_url>/oauth2/token'

But since a user won't have access to the client_secret this is something that will need to be done server side and there needs to be an API exposed to the user to do that...