feat: add User-Agent request header

[bednar]

Closes #59

• CHANGELOG.md updated
• Rebased/mergeable
• Tests pass
• Sign CLA (if not already signed)

[codecov-io]

Codecov Report

Merging #60 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #60   +/-   ##
=======================================
  Coverage   87.88%   87.88%           
=======================================
  Files          22       22           
  Lines        1709     1709           
=======================================
  Hits         1502     1502           
  Misses        207      207

Impacted Files	Coverage Δ
influxdb_client/api_client.py	67.3% <100%> (ø)	⬆️

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.88%. Comparing base (716b6b3) to head (6ac9731).
Report is 351 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master      #60   +/-   ##
=======================================
  Coverage   87.88%   87.88%           
=======================================
  Files          22       22           
  Lines        1709     1709           
=======================================
  Hits         1502     1502           
  Misses        207      207           

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