Removed library imports from the worker

[gavin-aguiar]

Removed library imports from the worker and only imported them in the function that they are used.

This should fix the users who have pinned their azure functions to a version older than 1.10.1.

Error seen for users with older pinned azure functions version
ERROR: unhandled error in functions worker,ImportError: cannot import name 'DataType' from 'azure.functions'

Description

Fixes #

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PR information

• The title of the PR is clear and informative.
• There are a small number of commits, each of which has an informative message. This means that previously merged commits do not appear in the history of the PR. For information on cleaning up the commits in your pull request, see this page.
• If applicable, the PR references the bug/issue that it fixes in the description.
• New Unit tests were added for the changes made and CI is passing.

Quality of Code and Contribution Guidelines

• I have read the contribution guidelines.

[codecov]

Codecov Report

Merging #1046 (8a80fbf) into dev (bcda54d) will increase coverage by 50.84%.
The diff coverage is 100.00%.

❗ Current head 8a80fbf differs from pull request most recent head b5c6caf. Consider uploading reports for the commit b5c6caf to get more accurate results

@@             Coverage Diff             @@
##              dev    #1046       +/-   ##
===========================================
+ Coverage   31.97%   82.81%   +50.84%     
===========================================
  Files          33       33               
  Lines        1764     1775       +11     
  Branches      282      282               
===========================================
+ Hits          564     1470      +906     
+ Misses       1199      235      -964     
- Partials        1       70       +69     

Flag	Coverage Δ
unittests	82.81% <100.00%> (+50.84%)	⬆️

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

Impacted Files	Coverage Δ
azure_functions_worker/functions.py	96.04% <100.00%> (+71.88%)	⬆️
azure_functions_worker/loader.py	84.52% <100.00%> (+51.19%)	⬆️
azure_functions_worker/constants.py	100.00% <0.00%> (ø)
...ed_memory_data_transfer/shared_memory_constants.py	100.00% <0.00%> (ø)
azure_functions_worker/logging.py	40.90% <0.00%> (+4.54%)	⬆️
...ared_memory_data_transfer/file_accessor_factory.py	73.33% <0.00%> (+6.66%)	⬆️
azure_functions_worker/bindings/tracecontext.py	75.00% <0.00%> (+12.50%)	⬆️
azure_functions_worker/bindings/retrycontext.py	80.00% <0.00%> (+20.00%)	⬆️
azure_functions_worker/bindings/context.py	87.50% <0.00%> (+29.16%)	⬆️
...dings/shared_memory_data_transfer/file_accessor.py	84.37% <0.00%> (+31.25%)	⬆️
... and 17 more

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[YunchuWang]

@gavin-aguiar codecov failed, plz add some tests