impl(auth): trust boundaries module

[alvarowolfx]

Towards #4186

[codecov]

Codecov Report

❌ Patch coverage is 57.89474% with 56 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.35%. Comparing base (ee6e32d) to head (6f14f3b).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/auth/src/trust_boundary.rs	37.50%	50 Missing ⚠️
src/auth/src/credentials/external_account.rs	62.50%	3 Missing ⚠️
src/auth/src/credentials/mds.rs	80.00%	2 Missing ⚠️
src/auth/src/credentials/impersonated.rs	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4346      +/-   ##
==========================================
- Coverage   95.01%   94.35%   -0.67%     
==========================================
  Files         195      196       +1     
  Lines        7429     7545     +116     
==========================================
+ Hits         7059     7119      +60     
- Misses        370      426      +56     

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