Manage data directory as ContextVar instead of global

[gordonhart]

Rather than using a static/global data directory, use a ContextVar exposed via the DataDirectoryProvider class that allows for runtime setting of the data directory. This unblocks usage of different data directories for different endpoint calls, and opens the door in the future to allow users to specify their own data directory or directories if they e.g. want to run AutoArena locally using data stored on a mounted NFS.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.11765% with 3 lines in your changes missing coverage. Please review.

Project coverage is 96.86%. Comparing base (a8b3706) to head (f89e41c).

Files with missing lines	Patch %	Lines
autoarena/api/router.py	88.88%	1 Missing ⚠️
autoarena/api/utils.py	95.23%	1 Missing ⚠️
autoarena/judge/utils.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk      #84      +/-   ##
==========================================
+ Coverage   96.82%   96.86%   +0.03%     
==========================================
  Files          33       33              
  Lines        1419     1437      +18     
==========================================
+ Hits         1374     1392      +18     
  Misses         45       45              

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