fix(plugin-chart-echarts): boxplot groupby incorrect

[villebro]

🐛 Bug Fix
The post processing operation didn't include the groupby columns, resulting in NULL values on the x-axis. This PR fixes the bug, adds tests and adds type guards for all post processing rules.

Closes apache/superset#16951 , closes apache/superset#16985

AFTER

BEFORE

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[villebro]

groupby is deprecated on QueryObject and is ambigous with columns, hence we should use groupby from formData here, as it carries a specific meaning for the Boxplot chart.

[zhaoyongjie]

LGTM

[codecov]

Codecov Report

Merging #1448 (f3927cb) into master (6b8cda4) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1448      +/-   ##
==========================================
+ Coverage   30.43%   30.50%   +0.07%     
==========================================
  Files         497      498       +1     
  Lines       10012    10024      +12     
  Branches     1691     1692       +1     
==========================================
+ Hits         3047     3058      +11     
- Misses       6719     6720       +1     
  Partials      246      246              

Impacted Files	Coverage Δ
...ui-chart-controls/src/operators/boxplotOperator.ts	100.00% <100.00%> (ø)
...superset-ui-core/src/query/types/PostProcessing.ts	100.00% <100.00%> (ø)
...lugin-chart-echarts/src/Timeseries/transformers.ts	43.36% <0.00%> (-0.39%)	⬇️
...ugins/plugin-chart-echarts/src/Timeseries/types.ts	100.00% <0.00%> (ø)
...s/legacy-plugin-chart-country-map/src/countries.ts	0.00% <0.00%> (ø)
.../plugin-chart-echarts/src/MixedTimeseries/types.ts	100.00% <0.00%> (ø)
...hart-echarts/src/MixedTimeseries/transformProps.ts	0.00% <0.00%> (ø)

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