Add new authorizer for testing authenticated http zarr access

[tcompa]

This would be helpful for fractal-analytics-platform/ngio#9, but we can implement it elsewhere if it "pollutes" fractal-vizarr-viewer too much.

---

The authorizer would be very basic:

• it knows about a single "user" via their ID and password, which can be exposed through env variables or even hardcoded (let's make sure that the authorizer and the AUTHORIZATION_SCHEME option both have clear names to specify they are for testing purposes).
• it receives a http request for data and authorizes it with basicAuth