Add helpers to work with service accounts for OIDC

[creydr]

Fixes #7277

Proposed Changes

• 🎁 Helpers to work with service accounts for OIDC

Pre-review Checklist

• At least 80% unit test coverage
• E2E tests for any new behavior
• Docs PR for any user-facing impact
• Spec PR for any new API feature
• Conformance test for any change to the spec

Hints for reviewers:

• the naming pattern can be adjusted (e.g. if we see they get too long), but this should help to get started with tasks like Support auto generation of Triggers identity service account and expose in AuthStatus #7222 or Support auto generation of PingSource identity service account and expose in AuthStatus #7227.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

❗ No coverage uploaded for pull request base (main@401068c). Click here to learn what that means.
Report is 17 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7278   +/-   ##
=======================================
  Coverage        ?   77.58%           
=======================================
  Files           ?      250           
  Lines           ?    13431           
  Branches        ?        0           
=======================================
  Hits            ?    10421           
  Misses          ?     2485           
  Partials        ?      525           

Files	Coverage Δ
pkg/auth/serviceaccount.go	100.00% <100.00%> (ø)

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[creydr]

@matzew could you recheck?

[matzew]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: creydr, matzew

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