fix: allow creating option in table select in dataset add modal

[nytai]

SUMMARY

There is currently an issue with adding google sheets as datasets. The dataset add modal expects there to be a schema selected before table select is enabled. Since google sheets don't have schemas there is no way to add a google sheet as a dataset. The fix is:

• Allow creating option in dataset add modal. Google sheets urls can now be pasted in the select field.
• schema is now optional. Simply leave it blank to add a google sheet.

Trade Offs: there will now be a higher possibility for errors as users will be able to submit the form without shema and the table option now allows input so users can paste non-existent tables into the select field. Long term we should consider an api that can tell us if a database 1. supports schemas, 2. allows free-form table inputs

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

• can add a google sheet as a dataset.
• can add a regular table as a dataset
• unit tests pass.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI (behind ENABLE_REACT_CRUD_VIEWS flag)
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-commenter]

Codecov Report

Merging #10369 into master will increase coverage by 0.74%.
The diff coverage is 62.50%.

@@            Coverage Diff             @@
##           master   #10369      +/-   ##
==========================================
+ Coverage   69.70%   70.45%   +0.74%     
==========================================
  Files         196      603     +407     
  Lines       18950    32431   +13481     
  Branches        0     3294    +3294     
==========================================
+ Hits        13210    22848    +9638     
- Misses       5740     9477    +3737     
- Partials        0      106     +106     

Flag	Coverage Δ
#cypress	55.38% <66.66%> (?)
#javascript	59.33% <62.50%> (?)
#python	69.78% <ø> (+0.07%)	⬆️

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

Impacted Files	Coverage Δ
...frontend/src/views/datasetList/AddDatasetModal.tsx	57.14% <0.00%> (ø)
superset-frontend/src/components/TableSelector.jsx	85.81% <83.33%> (ø)
superset/utils/pandas_postprocessing.py	76.96% <0.00%> (-12.98%)	⬇️
superset/errors.py	93.75% <0.00%> (-6.25%)	⬇️
superset/exceptions.py	95.12% <0.00%> (-4.88%)	⬇️
superset/views/base.py	72.50% <0.00%> (-0.92%)	⬇️
superset/viz.py	57.04% <0.00%> (-0.34%)	⬇️
superset/viz_sip38.py	0.00% <0.00%> (ø)
superset/models/slice.py	84.65% <0.00%> (ø)
superset/charts/schemas.py	100.00% <0.00%> (ø)
... and 421 more

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[pkdotson]

LGTM!