fix: dataset extra import/export

[betodealmeida]

SUMMARY

The extra attribute from datasets is being imported as a string. The resulting YAML looks like this:

extra: '{{\"warning_markdown\": \"*WARNING*\"}}'

When it should look like this:

extra:
  warning_markdown: '*WARNING*'

I fixed the export to run json.load on the field before the export, and fixed the import to support both formats.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TESTING INSTRUCTIONS

I added unit tests covering the export, and imports for the old and new format.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• 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 #17740 (22c80a9) into master (82b47ca) will decrease coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #17740      +/-   ##
==========================================
- Coverage   67.71%   67.61%   -0.11%     
==========================================
  Files        1605     1605              
  Lines       64198    64218      +20     
  Branches     6790     6790              
==========================================
- Hits        43474    43418      -56     
- Misses      18868    18944      +76     
  Partials     1856     1856              

Flag	Coverage Δ
hive	?
mysql	82.18% <100.00%> (+0.01%)	⬆️
postgres	82.24% <100.00%> (+0.01%)	⬆️
python	82.32% <100.00%> (-0.25%)	⬇️
sqlite	81.92% <100.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
superset/datasets/commands/export.py	87.50% <100.00%> (ø)
superset/datasets/commands/importers/v1/utils.py	58.33% <100.00%> (ø)
superset/datasets/schemas.py	96.92% <100.00%> (+0.31%)	⬆️
superset/db_engines/hive.py	0.00% <0.00%> (-85.19%)	⬇️
superset/db_engine_specs/hive.py	70.27% <0.00%> (-16.99%)	⬇️
superset/views/database/mixins.py	81.03% <0.00%> (-1.73%)	⬇️
superset/utils/webdriver.py	78.65% <0.00%> (-1.14%)	⬇️
superset/db_engine_specs/presto.py	83.50% <0.00%> (-0.84%)	⬇️
superset/dashboards/dao.py	95.55% <0.00%> (-0.75%)	⬇️
superset/db_engine_specs/base.py	88.29% <0.00%> (-0.39%)	⬇️
... and 12 more

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[eschutho]

I just left a nit question