feat: support preheat with self-signed certs

[gaius-qi]

Description

Related Issue

Motivation and Context

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[chlins]

lgtm

[codecov]

Codecov Report

Attention: Patch coverage is 30.76923% with 9 lines in your changes missing coverage. Please review.

Project coverage is 51.85%. Comparing base (1afe79e) to head (8c172b4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
manager/job/preheat.go	30.00%	7 Missing ⚠️
manager/job/job.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3541      +/-   ##
==========================================
- Coverage   51.88%   51.85%   -0.03%     
==========================================
  Files         190      190              
  Lines       20483    20488       +5     
==========================================
- Hits        10628    10625       -3     
- Misses       9047     9053       +6     
- Partials      808      810       +2     

Flag	Coverage Δ
unittests	51.85% <30.76%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
manager/config/config.go	96.33% <100.00%> (-0.04%)	⬇️
manager/job/job.go	16.00% <0.00%> (ø)
manager/job/preheat.go	50.91% <30.00%> (-0.75%)	⬇️

... and 1 file with indirect coverage changes