Use official secretreader plugin

[kahirokunn]

Fixes #

Background

The multicluster e2e setup previously built and used a test-only exec credential plugin because the official Cluster Inventory API access provider plugins were not available yet. The official secretreader and kubeconfig-secretreader plugin images have now been released, so this PR switches the examples and tests to use those published images directly.

Proposed Changes

• Switch the multicluster access provider examples and e2e setup from the test-only self-built exec plugin to the official Cluster Inventory API secretreader plugin images.

Release Note

Multicluster access provider examples and e2e setup now use the official Cluster Inventory API secretreader plugin images.

[knative-prow]

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This pull-request has been approved by: kahirokunn
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.84%. Comparing base (097eae0) to head (53e2f06).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2306   +/-   ##
=======================================
  Coverage   63.84%   63.84%           
=======================================
  Files          55       55           
  Lines        2478     2478           
=======================================
  Hits         1582     1582           
  Misses        777      777           
  Partials      119      119           

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