fix(crud): set default extra value when adding database

[villebro]

SUMMARY

When adding a new database via the new React CRUD, the connection times out due to the extra field not being prepopulated with default values (they were only shown as a placeholder).

TEST PLAN

Local testing

ADDITIONAL INFORMATION

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

[codecov-io]

Codecov Report

Merging #11262 into master will decrease coverage by 4.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #11262      +/-   ##
==========================================
- Coverage   65.57%   61.38%   -4.20%     
==========================================
  Files         831      831              
  Lines       39514    39512       -2     
  Branches     3598     3598              
==========================================
- Hits        25913    24255    -1658     
- Misses      13492    15075    +1583     
- Partials      109      182      +73     

Flag	Coverage Δ
#cypress	?
#javascript	62.66% <100.00%> (+<0.01%)	⬆️
#python	60.62% <ø> (-0.14%)	⬇️

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

Impacted Files	Coverage Δ
...end/src/views/CRUD/data/database/DatabaseModal.tsx	60.90% <100.00%> (-10.31%)	⬇️
superset-frontend/src/SqlLab/App.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/explore/App.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/dashboard/App.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/explore/index.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/dashboard/index.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/setup/setupColors.js	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/chart/ChartContainer.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/setup/setupFormatters.js	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/explore/reducers/index.js	0.00% <0.00%> (-100.00%)	⬇️
... and 176 more

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