Make datetime fieldtypes timezone aware

[yunzheng]

This PR will add the following:

• Record datetime fields are now offset-aware by default
• Naive datetime fields are converted to UTC
• Support for packing/unpacking aware datetimes

Note that comparing to naive datetime objects will now break and is also in line with default Python behaviour

To ensure uniform datetime field output they are always displayed in UTC
To disable this behaviour you can set the environment variable FLOW_RECORD_TZ=NONE

[codecov]

Codecov Report

Merging #78 (796f150) into main (464914e) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #78      +/-   ##
==========================================
+ Coverage   78.86%   79.04%   +0.18%     
==========================================
  Files          32       32              
  Lines        2862     2878      +16     
==========================================
+ Hits         2257     2275      +18     
+ Misses        605      603       -2     

Flag	Coverage Δ
unittests	79.04% <100.00%> (+0.18%)	⬆️

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

Files Changed	Coverage Δ
flow/record/adapter/elastic.py	0.00% <ø> (ø)
flow/record/base.py	90.30% <100.00%> (+0.01%)	⬆️
flow/record/fieldtypes/__init__.py	92.10% <100.00%> (+0.76%)	⬆️
flow/record/jsonpacker.py	91.25% <100.00%> (ø)
flow/record/packer.py	95.28% <100.00%> (+0.04%)	⬆️
flow/record/stream.py	94.87% <100.00%> (ø)

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