fix(identity): update endpoint for Create/UpdateWorkloadIdentity

[spatch1923]

Issue #, if available:

Description of changes:

This change updates the identity client to use the control plane endpoint for Create/UpdateWorkloadIdentity.

The Create/UpdateWorkloadIdentity functions were originally set up to use clients using the AgentCore data plane endpoint to avoid an old issue with local development. This issue has been fixed already, so these functions should use the control plane endpoint like the rest of the control plane operations.

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[jariy17]

Please expand on this issue:
The Create/UpdateWorkloadIdentity functions were originally set up to use clients using the AgentCore data plane endpoint to avoid an old issue with local development.
What were original problem? Why was the hack introduce?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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##             main     #249   +/-   ##
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  Coverage        ?   90.52%           
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  Files           ?       35           
  Lines           ?     3421           
  Branches        ?      511           
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  Hits            ?     3097           
  Misses          ?      179           
  Partials        ?      145           

Flag	Coverage Δ
unittests	90.52% <100.00%> (?)

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