fix(mssql): support cte in virtual tables

[sujiplr]

SUMMARY

Superset was not allowing to run CTE based SQLs on MSSQL while creating virtual tables, because MSSQL is not supporting CTE in subquery. Always the CTE sql should be at the top. Hence added the fix to make the SQL to run as same as CTE, this feature can be used for all the databases. Added a new flag called "allows_cte_in_subquery" , if it's True, Superset will parse CTE as Subquery and run it else Superset will run the CTE as same as it is.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

Added unit test case, unit test case can be run against mssql dbengine using below command
tox -e sqlite tests/unit_tests/db_engine_specs/test_mssql.py

[codecov]

Codecov Report

Merging #18567 (6a47c7d) into master (2929bb1) will increase coverage by 0.00%.
The diff coverage is 96.96%.

@@           Coverage Diff           @@
##           master   #18567   +/-   ##
=======================================
  Coverage   66.32%   66.33%           
=======================================
  Files        1592     1592           
  Lines       62569    62596   +27     
  Branches     6295     6295           
=======================================
+ Hits        41501    41525   +24     
- Misses      19416    19419    +3     
  Partials     1652     1652           

Flag	Coverage Δ
hive	52.08% <60.60%> (-0.06%)	⬇️
mysql	81.25% <96.96%> (+<0.01%)	⬆️
postgres	81.30% <96.96%> (+<0.01%)	⬆️
presto	51.92% <60.60%> (-0.06%)	⬇️
python	81.74% <96.96%> (+<0.01%)	⬆️
sqlite	81.00% <96.96%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
superset/connectors/sqla/models.py	88.32% <94.11%> (-0.20%)	⬇️
superset/db_engine_specs/base.py	88.74% <100.00%> (+0.31%)	⬆️
superset/db_engine_specs/mssql.py	95.91% <100.00%> (+0.08%)	⬆️
superset/charts/schemas.py	99.34% <0.00%> (ø)

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[villebro]

LGTM, thanks for working on this, a huge improvement to support CTEs on databases that don't support nested CTEs!

[sujiplr]

Thank you @villebro for all the help you provided .