chore(ssh-tunnel): Refactor establishing raw connection with contextmanger

[hughhhh]

SUMMARY

Refactoring how we instantiate raw connections with the new @contextmanager pattern for instantiating sqlalchemy engines. With this PR, we can do the samething but with raw connections within the engine. Instead of instantiating the 2 contextmangers to get access to the connections users of this functions will only have make one call to get the proper context.

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 #22366 (1e1b34c) into master (b6d39d1) will increase coverage by 0.00%.
The diff coverage is 81.42%.

@@           Coverage Diff           @@
##           master   #22366   +/-   ##
=======================================
  Coverage   66.91%   66.91%           
=======================================
  Files        1851     1851           
  Lines       70715    70709    -6     
  Branches     7766     7766           
=======================================
- Hits        47320    47316    -4     
+ Misses      21373    21371    -2     
  Partials     2022     2022           

Flag	Coverage Δ
hive	52.46% <62.85%> (+<0.01%)	⬆️
javascript	53.85% <ø> (ø)
mysql	77.95% <81.42%> (+<0.01%)	⬆️
postgres	78.03% <81.42%> (+<0.01%)	⬆️
presto	52.36% <62.85%> (+<0.01%)	⬆️
python	81.27% <81.42%> (+<0.01%)	⬆️
sqlite	76.49% <81.42%> (+<0.01%)	⬆️
unit	51.19% <2.85%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
superset/db_engine_specs/base.py	89.25% <0.00%> (+0.12%)	⬆️
superset/db_engine_specs/gsheets.py	76.29% <0.00%> (+0.38%)	⬆️
superset/sql_validators/presto_db.py	82.43% <ø> (ø)
superset/sql_lab.py	81.13% <85.71%> (-0.08%)	⬇️
superset/connectors/sqla/utils.py	89.00% <100.00%> (-0.33%)	⬇️
superset/db_engine_specs/presto.py	88.02% <100.00%> (-0.08%)	⬇️
superset/models/core.py	90.61% <100.00%> (+0.10%)	⬆️

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[betodealmeida]

@hughhhh would you mind adding at least a summary to the PR?

[betodealmeida]

Nice refactoring, it's a good clean up!

Let's just add a summary so that anyone who revisits this PR in the future has some context.