chore(tags): Refactor logic to leverage Flask-SQLAlchemy extension

[john-bodley]

SUMMARY

Rather than passing in the engine—which isn't apparent whether it refers to an external engine or Superset's metadata engine—when managing tags, this PR simply leverages the Flask-SQLAlchemy extension directly.

Note it's not overly clear whether we should use db.engine.execute(...) or db.session.execute(...), though per here it seems like the later is preferred when using the ORM functionality (as opposed to raw SQL).

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

CI.

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 #21459 (e27006b) into master (8d1b7ec) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master   #21459   +/-   ##
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  Coverage   66.82%   66.82%           
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  Files        1799     1799           
  Lines       68831    68830    -1     
  Branches     7332     7332           
=======================================
  Hits        45997    45997           
+ Misses      20954    20953    -1     
  Partials     1880     1880           

Flag	Coverage Δ
hive	52.89% <0.00%> (+<0.01%)	⬆️
javascript	53.15% <ø> (ø)
mysql	78.21% <0.00%> (+<0.01%)	⬆️
postgres	78.27% <0.00%> (+<0.01%)	⬆️
presto	52.79% <0.00%> (+<0.01%)	⬆️
python	81.42% <0.00%> (+<0.01%)	⬆️
sqlite	76.77% <0.00%> (+<0.01%)	⬆️
unit	50.92% <0.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
superset/cli/update.py	0.00% <0.00%> (ø)
superset/common/tags.py	0.00% <0.00%> (ø)

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