Add thread_local_storage property to Context

[EvanR-Dev]

In addition to: Azure/azure-functions-python-worker#1171

Added property to Library.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (dev@847f763). Click here to learn what that means.
The diff coverage is 83.33%.

❗ Current head bba8884 differs from pull request most recent head fc6c6b0. Consider uploading reports for the commit fc6c6b0 to get more accurate results

@@          Coverage Diff           @@
##             dev     #167   +/-   ##
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  Coverage       ?   90.35%           
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  Files          ?       53           
  Lines          ?     3037           
  Branches       ?      805           
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  Hits           ?     2744           
  Misses         ?      216           
  Partials       ?       77           

Flag	Coverage Δ
unittests	90.31% <83.33%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
azure/functions/_http_asgi.py	90.32% <ø> (ø)
azure/functions/_abc.py	75.89% <80.00%> (ø)
azure/functions/_http_wsgi.py	91.08% <100.00%> (ø)

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