Add header auth module

[himanshugoel2797]

Some authentication methods like Shibboleth can sit in front of the server and provide the authenticated username via the reverse proxy. We can utilize a Basic HTTP authorization header to pass this into Sirepo, in a similar manner to the existing basic authentication.

This requires some care on the reverse proxy (unsetting any incoming authorization headers and replacing them with the username from Shibboleth). A token requirement has been added to prevent this authentication method from working without properly configuring the reverse proxy.

This module is very similar to the basic auth module, so, maybe it should be integrated into there with some configuration variables?