xds/clusterimpl: update TestChildPolicyChangeOnConfigUpdate to use custom lb policy.

[Pranjali-2501]

Fixes #8703

Description

This PR fixes the data race in TestChildPolicyChangeOnConfigUpdate.

Changes

• Isolated Policy: Instead of overwriting the global pick_first policy (and triggering a race condition via Unregister), the test now registers a unique custom stub policy named test_pick_first.
• TypedStruct Configuration: Updated the management server resource update to use v3xdsxdstypepb.TypedStructand specify the custom type.googleapis.com/test_pick_first policy via TypeURL.

RELEASE NOTES: N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.21%. Comparing base (69769dc) to head (b1b7bfa).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8730      +/-   ##
==========================================
+ Coverage   83.16%   83.21%   +0.04%     
==========================================
  Files         419      419              
  Lines       32450    32450              
==========================================
+ Hits        26988    27002      +14     
+ Misses       4068     4060       -8     
+ Partials     1394     1388       -6     

see 20 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[easwars]

Please ensure that you can run this test 10K or 100K times on Forge before merging this. You could use the instructions here to pull in this change into a piper client and run the test with the race detector turned on (10K/100K times). Thanks.