refactor: expose the export annotation util

[zhiying-lin]

What type of PR is this?
/kind feature

What this PR does / why we need it:
Expose this util so that fleet repo can use it to validate the weight annotation.

Which issue(s) this PR fixes:

Fixes #

Requirements:

• run make reviewable for basic local test
• Read and followed fleet-networking's Code of conduct.
• uses conventional commit messages
• includes documentation
• adds unit tests

How has this code been tested

e2e test

Special notes for your reviewer

ExtractWeightFromServiceExport does not return the User error anymore, and it will depend on the caller to wrap it. So that it can avoid the loop dep.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.13%. Comparing base (aeabb6d) to head (af05501).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #271      +/-   ##
==========================================
+ Coverage   79.94%   80.13%   +0.18%     
==========================================
  Files          28       29       +1     
  Lines        3985     3986       +1     
==========================================
+ Hits         3186     3194       +8     
+ Misses        638      631       -7     
  Partials      161      161              

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