terminalDrobox: implement automatic request of a new auth token

[iarovikov]

Check if Dropbox OAuth implementation supports refresh tokens according to the OAuth 2 specification.

if it does, implement automatic request of a new authorization token using the refresh token when the existing token gets invalidated or expired.

You can save refresh token to AuthorizationTokenDO#AdditionalAttributes in the form refresh_token=... See terminalSalesforce.Infrastructure.Authentication#RefreshAccessToken for an example of implementation.