feat: add new backend cesid

[vyskocilpavel]

Add new backend for Czech Educational and Scientific Identification AAI service

[codecov]

Codecov Report

❌ Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.17%. Comparing base (2605731) to head (c8deeca).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
social_core/backends/cesid.py	66.66%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1990      +/-   ##
==========================================
- Coverage   89.19%   89.17%   -0.02%     
==========================================
  Files         345      347       +2     
  Lines       13926    13945      +19     
  Branches      698      698              
==========================================
+ Hits        12421    12436      +15     
- Misses       1262     1266       +4     
  Partials      243      243              

Flag	Coverage Δ
unittests	89.17% <78.94%> (-0.02%)	⬇️

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