Add codecov as github action.

[k4leung4]

Signed-off-by: Kenny Leung kleung@chainguard.dev

Summary

This adds codecov as the code coverage tool for PRs.
Similar to what is being done in rekor in sigstore/rekor#676

Resurrecting #430 @nsmith5

Ticket Link

Fixes

Release Note

None

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@6a12159). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #449   +/-   ##
=======================================
  Coverage        ?   45.66%           
=======================================
  Files           ?       14           
  Lines           ?      957           
  Branches        ?        0           
=======================================
  Hits            ?      437           
  Misses          ?      460           
  Partials        ?       60           

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[haydentherapper]

@k4leung4 When I click on the links in the codecov comment, I only see errors about missing pages, such as https://app.codecov.io/gh/sigstore/fulcio/compare/449?src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr%20comments&utm_term=None