feat: allow to configure a connection pool maxsize

[bednar]

Closes #204

Proposed Changes

Allow to configure a connection pool maxsize.

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]

Codecov Report

Merging #215 (bc63fe6) into master (5e56938) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
+ Coverage   89.94%   90.07%   +0.12%     
==========================================
  Files          26       26              
  Lines        1969     1974       +5     
==========================================
+ Hits         1771     1778       +7     
+ Misses        198      196       -2     

Impacted Files	Coverage Δ
influxdb_client/client/influxdb_client.py	99.27% <100.00%> (+0.78%)	⬆️
influxdb_client/rest.py	81.81% <0.00%> (+0.75%)	⬆️

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