back end public access

[tcnichol]

Allow access to public datasets, files and metadata through the back end api.

[tcnichol]

Right now, this is not possible because the router for datasets is protected by
dependencies=[Depends(get_current_username)]

My solution is to create a new dependency that would get the current username OR return an anonymous user, like in clowder1. In datasets, files and metadata, the get routes would use the new method, while all others would use the old method.