Add support for passing Authorization header

[najose]

Supporting Authorization header would allow some Quetz setups that use proxies in front of it for extra security to allow authenticated access to Quetz.

One example is an Identity-Aware Proxy that looks for the Authroization header for a bearer token. So without this header all calls to Quetz gets dropped or redirected.

Few ways I can think of to implement this:

• Look at .mamba/auth/authentication.json for possible auth tokens
• Or expose an environment variable that if set will pass along it's value as the value for Authorization header.