feat: load the default extension collection lazily

[scgkiran]

This change replaces extensions.DefaultCollection with extensions.GetDefaultCollection()

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 63.77%. Comparing base (3be7ba3) to head (513f32b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
extensions/extension_mgr.go	61.29%	8 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
- Coverage   63.83%   63.77%   -0.07%     
==========================================
  Files          44       44              
  Lines       10893    10904      +11     
==========================================
  Hits         6954     6954              
- Misses       3628     3636       +8     
- Partials      311      314       +3     

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[EpsilonPrime]

Thanks for cleaning this up!

Suggestion for alternative PR title:
feat: make the default extension collection load lazily

[EpsilonPrime]

Looks good, thanks!