Add support for validating request against dynamic scopes

The first implementation of the ScitokensChecker uses a fixed list of scopes defined at initialisation. This works well for servers that validate each request against a fixed set of scopes (normally a set of one, e.g. service.read).

For more complex services, it would be good to allow specifying the required scope(s) at enforcement time, to allow using the same ScitokensChecker for different types of requests that require different scopes, e.g. service.read for GET /data, but service.create for POST /data.

This should be fairly easy to achieve by expanding the test method of the custom Enforcer to accept target scopes at runtime.