Add oauth token validation extension to autogen extensions

[yogitasrivastava]

Why are these changes needed?

This extension is needed so that token validation (access/jwt token) could be abstracted as a class that could be reused across AI agent applications for validating the token requested using autogen.extensions auth/azure

This extension standardizes the manner in which AI agent applications will validate the access token requested using autogen.extensions auth/azure

Related issue number

#6435

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

Attention: Patch coverage is 98.11321% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.57%. Comparing base (446da62) to head (3f831a7).

Files with missing lines	Patch %	Lines
...autogen_ext/oauthtokenvalidation/_jwt_validator.py	98.03%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6436      +/-   ##
==========================================
+ Coverage   79.52%   79.57%   +0.05%     
==========================================
  Files         225      227       +2     
  Lines       16661    16714      +53     
==========================================
+ Hits        13249    13301      +52     
- Misses       3412     3413       +1     

Flag	Coverage Δ
unittests	79.57% <98.11%> (+0.05%)	⬆️

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