[ios] Preparation for moving AuthenticationService{,Factory}

Add empty target which will eventually contain the code for the
AuthenticationService{,Factory}. This allow adding dependencies
on those target in the internal repository before the code is
moved there.

This is part of a multi-CLs change:
 => https://crrev.com/c/5961720
  - https://crrev.com/i/7766952
  - https://crrev.com/c/5961581
  - https://crrev.com/c/5961504
  - https://crrev.com/c/5961143
  - https://crrev.com/i/7775666
  - https://crrev.com/c/5962001
  - https://crrev.com/c/5961341
  - https://crrev.com/c/5961505
  - https://crrev.com/c/5961144
  - https://crrev.com/c/5961743
  - https://crrev.com/c/5961506

Bug: 325612973
Change-Id: If7a0665aeeb7e28a9cdfe07bab84e9c7b9740951
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5961720
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1374556}