fix: Re-add schema as a searchable field for the datasets v1 API

[john-bodley]

SUMMARY

This PR fixes a regression introduced in #20136 where at Airbnb an existing api/v1/dataset GET request broke which was filtering on the schema name, i.e.,

{
  "message": "Filter column: schema not allowed to filter"
}

The fix is to include schema in the search_columns.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

Verified that the API request succeeded.

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 #20711 (bc2ceb6) into master (5beb1aa) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #20711   +/-   ##
=======================================
  Coverage   66.83%   66.83%           
=======================================
  Files        1750     1750           
  Lines       65894    65894           
  Branches     7017     7017           
=======================================
  Hits        44041    44041           
  Misses      20067    20067           
  Partials     1786     1786           

Flag	Coverage Δ
hive	53.95% <100.00%> (ø)
javascript	51.94% <ø> (ø)
mysql	82.48% <100.00%> (ø)
postgres	82.55% <100.00%> (ø)
presto	53.81% <100.00%> (ø)
python	82.98% <100.00%> (ø)
sqlite	82.33% <100.00%> (ø)
unit	51.03% <100.00%> (ø)

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

Impacted Files	Coverage Δ
superset/datasets/api.py	88.52% <100.00%> (ø)

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[etr2460]

lgtm, thanks for the fix