[WIP] /files endpoint implementation

[ml-evs]

Related #1225

[codecov]

Codecov Report

Merging #1326 (5642038) into master (99f47ee) will decrease coverage by 0.30%.
The diff coverage is 68.00%.

@@            Coverage Diff             @@
##           master    #1326      +/-   ##
==========================================
- Coverage   91.49%   91.19%   -0.31%     
==========================================
  Files          72       75       +3     
  Lines        4364     4418      +54     
==========================================
+ Hits         3993     4029      +36     
- Misses        371      389      +18     

Flag	Coverage Δ
project	91.19% <68.00%> (-0.31%)	⬇️
validator	90.60% <68.00%> (-0.30%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
optimade/server/routers/files.py	0.00% <0.00%> (ø)
optimade/models/__init__.py	100.00% <100.00%> (ø)
optimade/models/files.py	100.00% <100.00%> (ø)
optimade/models/responses.py	97.87% <100.00%> (+0.25%)	⬆️
optimade/server/mappers/__init__.py	100.00% <100.00%> (ø)
optimade/server/mappers/files.py	100.00% <100.00%> (ø)
optimade/server/main_index.py	93.10% <0.00%> (-1.64%)	⬇️
optimade/server/routers/utils.py	97.24% <0.00%> (-0.89%)	⬇️
optimade/server/config.py	93.54% <0.00%> (+0.07%)	⬆️

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[JPBergsma]

I had already started to implement the Files endpoint as well. See my local branch https://github.com/JPBergsma/optimade-python-tools/tree/JPBergsma/add_files_endpoint
I had assigned myself to issue #1225, so I thought it would be clear I was planning to work on this.

[ml-evs]

I had already started to implement the Files endpoint as well. See my local branch https://github.com/JPBergsma/optimade-python-tools/tree/JPBergsma/add_files_endpoint I had assigned myself to issue #1225, so I thought it would be clear I was planning to work on this.

Ah okay, I should have checked, I just had some time to kill on the train. Feel free to take stuff from this PR into your branch and I'll close this.