feat: add support for serialization FluxTable into JSON

[bednar]

Closes #311

Proposed Changes

The result of query could be serialized into JSON by:

import json
from influxdb_client.client.flux_table import FluxStructureEncoder

output = json.dumps(tables, cls=FluxStructureEncoder, indent=2)
print(output)

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 #320 (e931ef9) into master (e2d7911) will decrease coverage by 0.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #320      +/-   ##
==========================================
- Coverage   92.53%   92.52%   -0.02%     
==========================================
  Files          27       27              
  Lines        2211     2220       +9     
==========================================
+ Hits         2046     2054       +8     
- Misses        165      166       +1     

Impacted Files	Coverage Δ
influxdb_client/client/flux_table.py	90.16% <88.88%> (-0.23%)	⬇️

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[rhajek]

Unfortunately, in Python there is no way to add a method to a class (like toJSON in JS).