Cache token when using Azure Identity

[alanrmatthews]

The Azure Identity credential provider provides a unix-style timestamp of when the token expires. Instead of pulling a new token every time, cache the existing token (if it exists) until the expiration so we don't need to keep acquiring new tokens.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@742ed16). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
simple_ado/auth/ado_azid_auth.py	50.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #55   +/-   ##
=======================================
  Coverage        ?   49.38%           
=======================================
  Files           ?       34           
  Lines           ?     2106           
  Branches        ?        0           
=======================================
  Hits            ?     1040           
  Misses          ?     1066           
  Partials        ?        0           

Flag	Coverage Δ
unittests	49.38% <50.00%> (?)

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