feat: add possibility to specify default timezone for datetimes without tzinfo

[bednar]

Closes #73

Proposed Changes

Added possibility to specify default timezone for datetimes without tzinfo.

It is useful for specify system timezone:

from dateutil.tz import tzlocal

from influxdb_client.client.util import date_utils
from influxdb_client.client.util.date_utils import DateHelper

date_utils.date_helper = DateHelper(timezone=tzlocal())

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• pytest tests completes successfully
• Commit messages are in semantic format
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Merging #238 (90639c3) into master (f698030) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #238   +/-   ##
=======================================
  Coverage   91.87%   91.88%           
=======================================
  Files          25       25           
  Lines        2031     2033    +2     
=======================================
+ Hits         1866     1868    +2     
  Misses        165      165           

Impacted Files	Coverage Δ
influxdb_client/client/util/date_utils.py	96.29% <100.00%> (+0.29%)	⬆️

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