koordlet: add extended HTTP handler registry

[saintube]

Ⅰ. Describe what this PR does

koordlet: add a registry for external HTTP handlers.

Ⅱ. Does this pull request fix one issue?

Ⅲ. Describe how to verify it

Ⅳ. Special notes for reviews

V. Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests
• All checks passed in make test

[jasonliu747]

/lgtm

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.72%. Comparing base (e64e74b) to head (d997f02).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2006      +/-   ##
==========================================
+ Coverage   68.70%   68.72%   +0.02%     
==========================================
  Files         421      421              
  Lines       38899    38899              
==========================================
+ Hits        26726    26734       +8     
+ Misses       9855     9852       -3     
+ Partials     2318     2313       -5     

Flag	Coverage Δ
unittests	68.72% <ø> (+0.02%)	⬆️

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[zwzhang0107]

/lgtm
/approve

[hormes]

/approve

[koordinator-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hormes, jasonliu747, zwzhang0107

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• cmd/OWNERS [hormes]

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