feat: add proxy_headers to configuration options

[bednar]

Closes #300
Closes #294

Proposed Changes

1. added proxy_headers to configuration options
2. added docs how to configure proxy

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 #301 (020a6cc) into master (cf21862) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #301   +/-   ##
=======================================
  Coverage   92.64%   92.65%           
=======================================
  Files          27       27           
  Lines        2189     2191    +2     
=======================================
+ Hits         2028     2030    +2     
  Misses        161      161           

Impacted Files	Coverage Δ
influxdb_client/rest.py	84.21% <ø> (ø)
influxdb_client/client/influxdb_client.py	98.13% <100.00%> (+0.01%)	⬆️
influxdb_client/configuration.py	83.87% <100.00%> (+0.17%)	⬆️

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