Add basic auth to schema registry

[claudio-benfatto]

This PR enables basic authentication for schema registry by the means of adding two new parameters to the root cli command:

• --username, -u: basic authentication user
• --password, -p: basic authentication password

Basic auth itself is implemented in a way to make it easy to support different kind of authentications based on the Authorization header, should they be supported in the future or by custom 3rd party implementations.