fix: adds TLS certificate validation option for SMTP

[dpgaspar]

SUMMARY

By default python's standard library will not validate SMTP server certificates, more info here:
context: python/cpython#91826

This PR adds a new config key name SMTP_SSL_SERVER_AUTH that enforces smtp server certificate validation,
using the default system CA certificates.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

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

Merging #21272 (4d53b9f) into master (6223042) will decrease coverage by 0.19%.
The diff coverage is 93.75%.

❗ Current head 4d53b9f differs from pull request most recent head 499e1ae. Consider uploading reports for the commit 499e1ae to get more accurate results

@@            Coverage Diff             @@
##           master   #21272      +/-   ##
==========================================
- Coverage   66.43%   66.23%   -0.20%     
==========================================
  Files        1784     1784              
  Lines       68185    68190       +5     
  Branches     7265     7265              
==========================================
- Hits        45298    45166     -132     
- Misses      21018    21155     +137     
  Partials     1869     1869              

Flag	Coverage Δ
hive	?
mysql	80.81% <100.00%> (+<0.01%)	⬆️
postgres	80.86% <100.00%> (+<0.01%)	⬆️
presto	?
python	80.92% <100.00%> (-0.42%)	⬇️
sqlite	?
unit	?

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

Impacted Files	Coverage Δ
...-frontend/src/components/FilterableTable/index.tsx	72.34% <50.00%> (ø)
superset/config.py	91.61% <100.00%> (+0.02%)	⬆️
superset/utils/core.py	90.27% <100.00%> (-0.07%)	⬇️
superset/db_engines/hive.py	0.00% <0.00%> (-85.19%)	⬇️
superset/db_engine_specs/hive.py	71.09% <0.00%> (-16.02%)	⬇️
superset/db_engine_specs/presto.py	81.32% <0.00%> (-6.63%)	⬇️
superset/db_engine_specs/sqlite.py	91.89% <0.00%> (-5.41%)	⬇️
superset/utils/celery.py	86.20% <0.00%> (-3.45%)	⬇️
superset/connectors/sqla/utils.py	86.36% <0.00%> (-2.28%)	⬇️
superset/models/sql_lab.py	74.90% <0.00%> (-2.00%)	⬇️
... and 8 more

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