Add support for request_delay in client and server

[ml-evs]

Supersedes #1420 and closes #1247 and closes #1418.

[codecov]

Codecov Report

Attention: Patch coverage is 86.36364% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.58%. Comparing base (6e9d171) to head (f8a35ec).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
optimade/client/client.py	80.00%	2 Missing ⚠️
optimade/server/config.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2237      +/-   ##
==========================================
- Coverage   90.58%   90.58%   -0.01%     
==========================================
  Files          75       75              
  Lines        4899     4917      +18     
==========================================
+ Hits         4438     4454      +16     
- Misses        461      463       +2     

Flag	Coverage Δ
project	90.58% <86.36%> (-0.01%)	⬇️
validator	90.58% <86.36%> (-0.01%)	⬇️

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