fix: column extra in import/export

[betodealmeida]

SUMMARY

When exporting a dataset, the extra attribute in columns was being exported as a plain string, instead of parsing the JSON. This would result in YAML like this:

- column_name: city
  ...
  extra: '{"foo": "bar"}'

Instead of:

- column_name: city
  ...
  extra:
    foo: bar

This PR:

1. Fixes the export, so that we get the more readable format.
2. Fixes the import, so that both formats are accepted.

(I also noted that the dataset extra attribute is not being parsed from JSON, and both import/export assume it's a string. I'll improve it in a next PR, exporting as an object and importing both formats.)

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TESTING INSTRUCTIONS

Added unit tests, testing that export is correct and import supports both formats.

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

[betodealmeida]

This logic was wrong, but since it's only used when loading examples (where the dataset DB is the examples DB) it works fine.

[codecov]

Codecov Report

Merging #17738 (1a01826) into master (0d2299c) will increase coverage by 0.01%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master   #17738      +/-   ##
==========================================
+ Coverage   68.06%   68.07%   +0.01%     
==========================================
  Files        1653     1653              
  Lines       66374    66374              
  Branches     7121     7121              
==========================================
+ Hits        45177    45185       +8     
+ Misses      19300    19292       -8     
  Partials     1897     1897              

Flag	Coverage Δ
hive	81.79% <60.00%> (+0.01%)	⬆️
mysql	82.16% <60.00%> (+0.01%)	⬆️
postgres	82.22% <60.00%> (+0.02%)	⬆️
python	82.56% <60.00%> (+0.02%)	⬆️
sqlite	81.90% <60.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
superset/datasets/commands/importers/v1/utils.py	58.33% <50.00%> (-0.50%)	⬇️
superset/datasets/commands/export.py	87.50% <71.42%> (+8.77%)	⬆️
superset/datasets/schemas.py	96.61% <100.00%> (ø)
superset/dashboards/commands/importers/v1/utils.py	81.42% <0.00%> (ø)
superset/reports/commands/execute.py	91.22% <0.00%> (+0.38%)	⬆️
superset/common/query_object.py	95.62% <0.00%> (+0.54%)	⬆️
superset/commands/importers/v1/utils.py	91.30% <0.00%> (+2.17%)	⬆️
superset/reports/commands/log_prune.py	89.28% <0.00%> (+3.57%)	⬆️

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