Core: Introduce enable/disable_iam_authentication context managers

[bblommers]

Fixes #9435

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.04%. Comparing base (6085b0b) to head (3b11745).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9487      +/-   ##
==========================================
- Coverage   93.04%   93.04%   -0.01%     
==========================================
  Files        1295     1295              
  Lines      116441   116464      +23     
==========================================
+ Hits       108342   108362      +20     
- Misses       8099     8102       +3     

Flag	Coverage Δ
servertests	28.89% <30.43%> (-0.01%)	⬇️
unittests	93.01% <100.00%> (-0.01%)	⬇️

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