Provide a more flexible authentication mechanism for a user request

[ykazakov]

Currently, the nbexchange API requests are secured with the cookie NAAS_JWT:

nbexchange/nbexchange/plugin/exchange.py

Line 49 in 985796a

jwt_token = os.environ.get("NAAS_JWT")

In the context of JupyterHub it would be nice to be able, instead, to authenticate request with a token.

See #148 for a concrete scenario.