feat: add 'extra' column to saved_queries GET

[mistercrunch]

SUMMARY

This column contains schedule information when the feature flag
SCHEDULED_QUERIES is enabled. Adding it here makes it available
through the REST API.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

manual

[betodealmeida]

+1

[codecov]

Codecov Report

Merging #13662 (c0727fe) into master (72d19b6) will increase coverage by 3.54%.
The diff coverage is 89.00%.

❗ Current head c0727fe differs from pull request most recent head 54ad48d. Consider uploading reports for the commit 54ad48d to get more accurate results

@@            Coverage Diff             @@
##           master   #13662      +/-   ##
==========================================
+ Coverage   77.41%   80.96%   +3.54%     
==========================================
  Files         918      310     -608     
  Lines       46673    25052   -21621     
  Branches     5720        0    -5720     
==========================================
- Hits        36132    20283   -15849     
+ Misses      10405     4769    -5636     
+ Partials      136        0     -136     

Flag	Coverage Δ
cypress	?
hive	80.25% <89.00%> (+<0.01%)	⬆️
javascript	?
mysql	?
postgres	?
presto	80.28% <87.00%> (-0.01%)	⬇️
python	80.96% <89.00%> (-0.15%)	⬇️
sqlite	80.22% <89.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
superset/queries/saved_queries/api.py	97.46% <ø> (ø)
superset/viz.py	56.07% <0.00%> (-0.04%)	⬇️
superset/common/query_actions.py	92.85% <75.00%> (-2.53%)	⬇️
superset/connectors/sqla/models.py	90.50% <89.06%> (-0.13%)	⬇️
superset/common/query_context.py	81.52% <100.00%> (-0.55%)	⬇️
superset/common/query_object.py	90.47% <100.00%> (-0.07%)	⬇️
superset/connectors/base/models.py	90.81% <100.00%> (+0.03%)	⬆️
superset/db_engine_specs/base.py	85.95% <100.00%> (-0.21%)	⬇️
superset/db_engine_specs/pinot.py	95.12% <100.00%> (+0.12%)	⬆️
superset/models/core.py	88.55% <100.00%> (-0.82%)	⬇️
... and 622 more

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[dpgaspar]

Would be nice to add this new field here: https://github.com/apache/superset/blob/master/tests/queries/saved_queries/api_tests.py#L133
I'll make another PR to make sure that adding a new field actually breaks a test (not just removing one)