Optimize the Python sample

[rayluo]

Reason for Change:

The previous sample was a proof-of-concept which only worked as a one-off script, and inefficiently. Problems include:

• Even if you rerun the script immediately, a new file I/O and new http I/O would incur.
• If a real app changes the previous sample into a long-lived daemon, the federation assertion would soon become outdated because the sample never reloads the assertion.

After this PR, the sample, if being used as a long-lived daemon, will automatically attempt token search from cache, and automatically reload the assertion when cache misses.

No.

No.

Requirements

• squashed commits
• included documentation
• added unit tests and e2e tests (if applicable).

Issue Fixed:

Please answer the following questions with yes/no:

Does this change contain code from or inspired by another project? If so, did you notify the maintainers and provide attribution?

• yes
• no

Notes for Reviewers:
I do not have an AKS environment to actually run this change. The repo owner shall be able to test that part.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.66%. Comparing base (94283c5) to head (660d364).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1269   +/-   ##
=======================================
  Coverage   51.66%   51.66%           
=======================================
  Files          53       53           
  Lines        2938     2938           
=======================================
  Hits         1518     1518           
  Misses       1378     1378           
  Partials       42       42           

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