[ios] Add a CopyDirectory method to session_internal_util

Add a function that recursively copies a directory (using hardlink
for individual files to reduce the amount of storage used) and add
unit tests.

Bug: 1383087
Change-Id: I3528554b5cf33882b59a480d48608376908495f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4938592
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Federica Germinario <fedegermi@google.com>
Cr-Commit-Position: refs/heads/main@{#1209401}