Add Config Flag To Pass Auth Header To ValidationHook

One use case is that you might want to only allow Athens to be "available" during a CI/CD build then locking it down once that build is done...one way to achieve this is that the Go can send an Auth header (right now only basic) to Athens, and then Athens can pass that header on the fly to the Validation hook and the hook will make sure the token is correct.

We should have this behind a flag because we should not be sending auth headers without a user's permission.

No files, tests, or entry points are named in the issue. Start by locating the existing basic-auth handling and ValidationHook configuration, then determine how an opt-in flag controls forwarding the header. Done means the auth header reaches the validation hook only when the flag is enabled.

Written by the indexing model from the issue text.