Client: attempt to switch to synchronous mode when existing event loop is detected

[ml-evs]

Related to #1939

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (6f76250) 90.79% compared to head (da842cf) 90.75%.

Files	Patch %	Lines
optimade/client/client.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1940      +/-   ##
==========================================
- Coverage   90.79%   90.75%   -0.05%     
==========================================
  Files          75       75              
  Lines        4728     4729       +1     
==========================================
- Hits         4293     4292       -1     
- Misses        435      437       +2     

Flag	Coverage Δ
project	90.75% <0.00%> (-0.05%)	⬇️
validator	90.67% <0.00%> (-0.05%)	⬇️

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