Function refresh

pub fn refresh(
    handler: &mut dyn Endpoint,
    request: &dyn Request,
) -> Result<BearerToken, Error>

Try to get a refreshed access token.

This has four basic phases:

1. Ensure the request is valid based on the basic requirements (includes required parameters)
2. Check any included authentication
3. Try to recover the refresh token 3.1. Check that it belongs to the authenticated client 3.2. If there was no authentication, assert token does not require authentication 3.3. Check the intrinsic validity (timestamp, scope)
4. Query the backend for a renewed (bearer) token