Add /files endpoint and entry type

[ml-evs]

Building from @JPBergsma's work, this PR adds the files endpoint and entry type.

This should not be merged until we have decided if we can support multiple OPTIMADE versions from a single optimade-python-tools version, but in the meantime, this can be used to generate the OpenAPI schemata for v1.2.

To-do:

• models
• server code
• OpenAPI schemas
• Testing
• Docs updates

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.30%. Comparing base (a2e25af) to head (71a5d05).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1414      +/-   ##
==========================================
+ Coverage   90.16%   90.30%   +0.14%     
==========================================
  Files          75       78       +3     
  Lines        5033     5086      +53     
==========================================
+ Hits         4538     4593      +55     
+ Misses        495      493       -2     

Flag	Coverage Δ
project	90.30% <100.00%> (+0.14%)	⬆️
validator	90.30% <100.00%> (+0.14%)	⬆️

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