Fix $HOME resolution and webhook namespace

[EngHabu]

Tracking issue

Describe your changes

• Automatically resolve $HOME references in the config file.
• Fix the webhook configuration namespace to use POD_NAMESPACE downstream API

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (0541851) 59.40% compared to head (0556293) 59.36%.

Files	Patch %	Lines
flytepropeller/pkg/webhook/init_cert.go	0.00%	6 Missing ⚠️
flytepropeller/pkg/webhook/config/config.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4509      +/-   ##
==========================================
- Coverage   59.40%   59.36%   -0.04%     
==========================================
  Files         638      638              
  Lines       53954    53957       +3     
==========================================
- Hits        32051    32033      -18     
- Misses      19374    19394      +20     
- Partials     2529     2530       +1     

Flag	Coverage Δ
unittests	59.36% <20.00%> (-0.04%)	⬇️

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[jeevb]

LGTM