feat: post-processing for pivot table v2

[betodealmeida]

SUMMARY

This PR adds the chart-specific post-processing for the Pivot Table v2, so that text-based reports match the data produced by the visualization.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TESTING INSTRUCTIONS

I added unit tests for pivot tables v1 and v2. I manually verified that the data produced by the functions match the data produced by the visualization.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #15879 (dfad33f) into master (3adf8e8) will increase coverage by 0.05%.
The diff coverage is 74.54%.

@@            Coverage Diff             @@
##           master   #15879      +/-   ##
==========================================
+ Coverage   76.83%   76.89%   +0.05%     
==========================================
  Files         988      988              
  Lines       52195    52249      +54     
  Branches     7095     7095              
==========================================
+ Hits        40106    40176      +70     
+ Misses      11864    11848      -16     
  Partials      225      225              

Flag	Coverage Δ
mysql	81.62% <74.54%> (+0.09%)	⬆️
postgres	81.64% <74.54%> (+0.13%)	⬆️
python	81.73% <74.54%> (+0.09%)	⬆️
sqlite	81.24% <74.54%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/charts/post_processing.py	80.00% <74.54%> (+65.36%)	⬆️
superset/charts/api.py	85.48% <0.00%> (ø)
superset/dashboards/api.py	92.95% <0.00%> (ø)

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