Custom authorization header functionality

[tomsseisums]

I have a setup where CraftCMS is used in fully headless fashion and our development/production host is behind Basic Auth.

Currently, if you're behind Basic Auth, it's impossible to auth with CraftQL, because it uses Authorization header value (as it is already used for Basic Auth).
Specifying multiple values, like Authorization: Basic <base64>, Bearer <token> is against spec.

Therefore, I implemented feature to provide custom Authorization header.