Provide opportunity to override `redirect_uri` in `Request.ltiAdvantageLogin`

[FElliger]

Is your feature request related to a problem? Please describe. When registering an LTI tool with the platform, the redirect URIs need to be fixed. I would like to set it to the lti.appRoute(). However when starting the login, ltijs puts the target_uri_link as the redirect_uri when sending the request to the platform's authentication endpoint.

Describe the solution you'd like Three options I can think of right now:

1. Always use lti.appRoute() as redirect_uri.
2. Provide a configuration option, e.g. options.useStaticLaunchUrl. If that's true then use lti.appRoute() as redirect_uri. Otherwise, use the current behavior.
3. Provide a middleware-like handler that can be implemented to hook into the auth request.

Favorite option would be 1.

Describe alternatives you've considered see above

Additional context Related code: https://github.com/Cvmcosta/ltijs/blob/master/src/Utils/Request.js#L16