fix: move columns to datasource object for bootstrap data

[hughhhh]

SUMMARY

Address concern after merge for adding data to datasource object instead of having it's own top level key

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

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 #20203 (3aca9d0) into master (7a2eb8b) will decrease coverage by 0.18%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master   #20203      +/-   ##
==========================================
- Coverage   66.46%   66.28%   -0.19%     
==========================================
  Files        1727     1727              
  Lines       64721    64723       +2     
  Branches     6823     6823              
==========================================
- Hits        43019    42901     -118     
- Misses      19970    20090     +120     
  Partials     1732     1732              

Flag	Coverage Δ
hive	?
mysql	82.15% <50.00%> (-0.01%)	⬇️
postgres	82.21% <50.00%> (-0.01%)	⬇️
presto	?
python	82.25% <50.00%> (-0.39%)	⬇️
sqlite	?
unit	?

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

Impacted Files	Coverage Δ
superset/views/core.py	77.34% <0.00%> (-0.02%)	⬇️
superset/models/sql_lab.py	91.08% <66.66%> (-0.48%)	⬇️
superset/db_engines/hive.py	0.00% <0.00%> (-85.19%)	⬇️
superset/db_engine_specs/hive.py	70.22% <0.00%> (-15.65%)	⬇️
superset/db_engine_specs/sqlite.py	91.89% <0.00%> (-5.41%)	⬇️
superset/db_engine_specs/presto.py	83.36% <0.00%> (-5.34%)	⬇️
superset/utils/celery.py	86.20% <0.00%> (-3.45%)	⬇️
superset/connectors/sqla/utils.py	88.75% <0.00%> (-2.50%)	⬇️
superset/result_set.py	96.85% <0.00%> (-1.58%)	⬇️
superset/connectors/sqla/models.py	88.41% <0.00%> (-1.19%)	⬇️
... and 5 more

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[eschutho]

would it be easier to make a columns property of the Query model that does this?