Add support for the Implicit Grant Flow to the HTTP Auth action

I've been working with HTTP Auth for some time now. As far as I know, HTTP Auth only supports the Authorized Grant Flow.

I know that the Implicit Grant Flow is less secure, but some APIs include additional functionality in the Implicit Grant Flow. For example, Fitbit's API requires re-authorization every 8 hours under the Authorized Grant Flow, but allows longer-lived tokens (up to one year) under the Implicit Grant Flow.

Thank you!