feat: support for custom parameters

[massi-ang]

Issue #, if available:
Resolves #109

Description of changes:
Added a new optional parameter in the request_access_token decorator to support passing custom parameters to the authorizer endpoint of the credential provider.

This allow to use the decorator with credential providers like Auth0

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@eee67da). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/bedrock_agentcore/services/identity.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #110   +/-   ##
=======================================
  Coverage        ?   89.46%           
=======================================
  Files           ?       23           
  Lines           ?     2306           
  Branches        ?      331           
=======================================
  Hits            ?     2063           
  Misses          ?      139           
  Partials        ?      104           

Flag	Coverage Δ
unittests	89.46% <90.00%> (?)

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