blob/auth: Teach it to handle HTTP Basic Auth too

There are cases when using user:password for /raw/... access is handy:

- when using query for auth (private_token) is not convenient for some
  reason (e.g. client processing software does not handle queries well
  when generating URLs)

- when we do not want to organize many artificial users and use their
  tokens, but instead just use per-project automatically setup

    gitlab-ci-token : <ci-token>

  artificial user & "password" which are already handled by auth backend
  for `git fetch` requests.

Handling is easy: if main auth backend rejects access, and there is
user:password in original request, we retry asking auth backend the way
as `git fetch` would do.

Access is granted if any of two ways to ask auth backend succeeds. This
way both private tokens / cookies and HTTP auth are supported.