fix(uploads): respect db engine spec's supports_multivalues_insert value for file uploads & enable multi-insert for MSSQL

[sfirke]

SUMMARY

This PR makes two changes to enable the setting of method = 'multi' when the Pandas function .to_sql is called to write a file upload to a data warehouse. This speeds up file uploads by 15x. See discussion #30152 for background and discussion.

Changes:

1. The base method df_to_sql does not currently access the supports_multivalues_insert attribute when it is set in a Superset EngineSpec. Instead it relies on this attribute being set in the SQLAlchemy implementation for that dialect. After this PR, Superset will check both of these places for the attribute.
2. Sets the attribute supports_multivalues_insert to True for Microsoft SQL Server (MSSQL), which has supported this behavior since SQL Server 2008.

Thanks to @nytai for advising.

TESTING INSTRUCTIONS

Check that uploading a flat file to a data warehouse succeeds. I tested it for MSSQL. This PR was written to not reduce any existing functionality (there's an or with the previous condition) but someone could check on another data warehouse too.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 83.69%. Comparing base (76d897e) to head (94550af).
Report is 724 commits behind head on master.

Additional details and impacted files

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##           master   #30222       +/-   ##
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  Branches     8568        0     -8568     
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- Hits        46114    32155    -13959     
+ Misses      28017     6265    -21752     
+ Partials     2105        0     -2105     

Flag	Coverage Δ
hive	48.87% <66.66%> (-0.29%)	⬇️
javascript	?
mysql	76.63% <100.00%> (?)
postgres	76.73% <100.00%> (?)
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sqlite	76.18% <100.00%> (?)
unit	60.44% <66.66%> (+2.82%)	⬆️

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[villebro]

LGTM, but all db engine spec class properites need to be also added to BaseEngineSpec with conservative defaults.

[villebro]

Generally LGTM but could be simplified more

[villebro]

Nice, really stoked about this PR, LGTM! 🚀