feat: introduce flag validate_user_defined_fields

[Vlad-Bukatin]

closes: #331

[codecov]

Codecov Report

❌ Patch coverage is 80.31496% with 75 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.29%. Comparing base (b8cb424) to head (0b8f197).

Files with missing lines	Patch %	Lines
crates/hotfix-message/src/builder.rs	83.39%	48 Missing ⚠️
crates/hotfix/src/session.rs	16.66%	15 Missing ⚠️
crates/hotfix/src/config.rs	78.94%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #352      +/-   ##
==========================================
- Coverage   91.72%   91.29%   -0.43%     
==========================================
  Files          94       94              
  Lines       10038    10372     +334     
==========================================
+ Hits         9207     9469     +262     
- Misses        831      903      +72     

Flag	Coverage Δ
core	91.18% <80.31%> (-0.44%)	⬇️

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