Authentication Custom Header should support Drupal token replacements

We started working on a patch for this, but ran into some other priorities. Will have to come back to this later if nobody else does.

I support the idea of using tokens and there may be other places where it could be useful as well.
But it means we will need the token module as a requirement right? (I didn't play with the token module in my modules since Drupal 7!) Not a big deal though...

I think most of that token functionality is in core now, except for the token browser (for that you need the Token module → ), but what I have in my head could be out-of-date. Actually this seems true as of Drupal 7! See https://www.drupal.org/documentation/modules/token → .

So no, we don't need to add that dependency.

Just a one-liner so far, but we should probably use dependency injection, injecting the `token` service into the RestClient.

Looks good to me. But it means we have to add the (core) token module as a dependency. I fixed tests accordingly. They pass on my environment but not on GitLab pipeline, probably because of the new dependency? Weird.

@colan feel free to merge when you thinks it is ok.