Extend certifier limits configuration to avoid OOMs

[poszu]

• limit maximum pending requests, reject too many with 429
• limit maximum request payload, reject too big with 413

[codecov]

Codecov Report

Attention: Patch coverage is 73.52941% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 94.18%. Comparing base (a868a7b) to head (c2b3a56).

Files	Patch %	Lines
certifier/src/certifier.rs	83.33%	5 Missing ⚠️
certifier/src/configuration.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #271      +/-   ##
==========================================
- Coverage   94.39%   94.18%   -0.21%     
==========================================
  Files          27       27              
  Lines        3352     3386      +34     
==========================================
+ Hits         3164     3189      +25     
- Misses        188      197       +9     

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