implement file server as a config flag

[Bud-Macaulay]

Seems to work in the limited testing.

Implements a new StaticFilesConfig entity and passes it into the ServerConfig.

I'm not sold on the naming conventions or the defaults. Perhaps could do with some language/docstrings for clarity aswell.

Happy to discuss

[Bud-Macaulay]

seems that the server config was leaking between tests - i presume due to some side effect when importing create_app. i have (hopefully) fixed this by isolating the tests i wrote in class fixtures

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.36%. Comparing base (d8b22c0) to head (01dba5f).

Files with missing lines	Patch %	Lines
optimade/server/config.py	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2419      +/-   ##
==========================================
+ Coverage   90.32%   90.36%   +0.04%     
==========================================
  Files          78       78              
  Lines        5086     5108      +22     
==========================================
+ Hits         4594     4616      +22     
  Misses        492      492              

Flag	Coverage Δ
project	90.36% <90.90%> (+0.04%)	⬆️
validator	90.36% <90.90%> (+0.04%)	⬆️

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[Bud-Macaulay]

Fixes #2417