dvcfs: implement get_file

[efiop]

Currently we use open, which is much slower. For example,

$ dvc get dvc-bench data/small/dataset

goes down from 110sec to 50sec (which is still too much).

Discovered as a part of #9098

[codecov]

Codecov Report

Base: 93.02% // Head: 92.99% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (464e904) compared to base (19038b3).
Patch coverage: 85.71% of modified lines in pull request are covered.

❗ Current head 464e904 differs from pull request most recent head 4fed069. Consider uploading reports for the commit 4fed069 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9057      +/-   ##
==========================================
- Coverage   93.02%   92.99%   -0.03%     
==========================================
  Files         457      457              
  Lines       36813    36827      +14     
  Branches     5338     5340       +2     
==========================================
+ Hits        34244    34246       +2     
- Misses       2050     2057       +7     
- Partials      519      524       +5     

Impacted Files	Coverage Δ
dvc/fs/dvc.py	96.42% <85.71%> (-0.64%)	⬇️
tests/unit/test_analytics.py	95.71% <0.00%> (-4.29%)	⬇️
dvc/analytics.py	90.14% <0.00%> (-2.82%)	⬇️
dvc/repo/experiments/queue/celery.py	85.92% <0.00%> (-1.86%)	⬇️

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[efiop]

There is a bit more so squeeze out in treeverse/dvc-objects#197