feat: load certs fallback to first key

[zirain]

Fallback to the first key when ca.crt is not found, similar to what we did in custom response body.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.70%. Comparing base (e78e268) to head (02dc923).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6327      +/-   ##
==========================================
+ Coverage   70.67%   70.70%   +0.02%     
==========================================
  Files         220      220              
  Lines       37003    37029      +26     
==========================================
+ Hits        26153    26181      +28     
+ Misses       9312     9311       -1     
+ Partials     1538     1537       -1     

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