add/commit: use hardlink to transfer files to the cache when cache.type is hardlink and on relink mode

[skshetry]

When the cache_type is hardlink, we can simply hardlink the file from the workspace to the cache, and we don't need to relink back from the cache to the workspace.

Name	Time (in s)
test_add_hardlink-add[hardlink-relink]	17.5393 (1.0)
test_add_hardlink-add[main]	32.9904 (1.88)

Note that this is a bit unsafe, because a failure in the middle of dvc add may end up in a situation where the file has been hardlinked to the cache, but we have not protected them yet to make it read-only. Modification of these files in the workspace might corrupt cache (since they are hardlinked). But this is what we were doing before #10513, and there is always a chance of corruption for hardlink anyway.

Also note that before #10513, on relink mode, we were hardlinking on all link types, but in this PR, I have limited it to just hardlink cache types.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.83%. Comparing base (f56343d) to head (9ef9024).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10557      +/-   ##
==========================================
+ Coverage   90.61%   90.83%   +0.22%     
==========================================
  Files         504      504              
  Lines       39582    39584       +2     
  Branches     5716     5716              
==========================================
+ Hits        35868    35958      +90     
+ Misses       3032     2965      -67     
+ Partials      682      661      -21     

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