chore(discovery): make the SDK use static discovery files

[beroussel]

GAE runtime will soon enough be decommissionned. To allow API discovery to continue working without breaking change, make the SDK rely on static discovery files hosted on LumApps CDN.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.81%. Comparing base (5628435) to head (0c5b966).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
lumapps/api/base_client.py	83.33%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
+ Coverage   85.64%   85.81%   +0.16%     
==========================================
  Files          16       16              
  Lines         843      853      +10     
  Branches      168      171       +3     
==========================================
+ Hits          722      732      +10     
  Misses         78       78              
  Partials       43       43              

Flag	Coverage Δ
unittests	85.81% <83.33%> (+0.16%)	⬆️

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Files with missing lines	Coverage Δ
lumapps/api/base_client.py	79.93% <83.33%> (+0.71%)	⬆️