Add configurable schema_url and index_schema_url options

[ml-evs]

Closes #1208

[codecov]

Codecov Report

Merging #1210 (6bd9f23) into master (a5691f7) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 6bd9f23 differs from pull request most recent head b4fd837. Consider uploading reports for the commit b4fd837 to get more accurate results

@@            Coverage Diff             @@
##           master    #1210      +/-   ##
==========================================
- Coverage   91.28%   91.27%   -0.01%     
==========================================
  Files          72       72              
  Lines        4289     4288       -1     
==========================================
- Hits         3915     3914       -1     
  Misses        374      374              

Flag	Coverage Δ
project	91.27% <100.00%> (-0.01%)	⬇️
validator	90.67% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
optimade/server/exception_handlers.py	84.28% <ø> (ø)
optimade/server/routers/index_info.py	100.00% <ø> (ø)
optimade/server/routers/utils.py	98.13% <ø> (ø)
optimade/server/config.py	91.57% <100.00%> (+0.18%)	⬆️
optimade/server/routers/info.py	95.65% <100.00%> (+0.19%)	⬆️
optimade/server/entry_collections/elasticsearch.py	97.40% <0.00%> (-0.13%)	⬇️

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[ml-evs]

I think I have addressed your concern @JPBergsma, so I will merge