impl(oauth2): use client factory for impersonation credentials

[coryan]

This will make things easier to test, I am thinking of oauth2_internal::MapCredentials() or oauth2_internal::GoogleDefaultCredentials(), which are not easy to mock at this time.

Part of the work for #5915

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This change is 

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Codecov Report

Base: 93.85% // Head: 93.86% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (b344dd2) compared to base (92d7528).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10416   +/-   ##
=======================================
  Coverage   93.85%   93.86%           
=======================================
  Files        1610     1610           
  Lines      145821   145829    +8     
=======================================
+ Hits       136866   136885   +19     
+ Misses       8955     8944   -11     

Impacted Files	Coverage Δ
...oud/internal/oauth2_minimal_iam_credentials_rest.h	100.00% <ø> (ø)
.../oauth2_impersonate_service_account_credentials.cc	100.00% <100.00%> (ø)
...ud/internal/oauth2_minimal_iam_credentials_rest.cc	94.59% <100.00%> (+22.22%)	⬆️
...ternal/oauth2_minimal_iam_credentials_rest_test.cc	100.00% <100.00%> (ø)
google/cloud/internal/unified_rest_credentials.cc	74.07% <100.00%> (+0.48%)	⬆️
...ernal/unified_rest_credentials_integration_test.cc	96.39% <100.00%> (+0.03%)	⬆️
google/cloud/internal/async_connection_ready.cc	89.36% <0.00%> (-4.26%)	⬇️
...bigtable/examples/bigtable_hello_instance_admin.cc	81.00% <0.00%> (-2.00%)	⬇️
google/cloud/storage/internal/curl_handle.cc	77.90% <0.00%> (-0.56%)	⬇️
google/cloud/completion_queue_test.cc	97.13% <0.00%> (-0.20%)	⬇️
... and 4 more

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