refactor: minor tunneling cleanup

[achettyiitr]

Description

• Avoid exposing unexported methods.
• Added test cases.
• Moved to the /warehouse/integrations package as it is meant to be used for integrations only ATM.

Linear Ticket

• Resolves PIPE-459

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (bd855f6) 71.43% compared to head (8047d5e) 71.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4034      +/-   ##
==========================================
+ Coverage   71.43%   71.45%   +0.01%     
==========================================
  Files         370      370              
  Lines       54632    54633       +1     
==========================================
+ Hits        39028    39036       +8     
+ Misses      13285    13279       -6     
+ Partials     2319     2318       -1     

Files	Coverage Δ
warehouse/integrations/postgres/postgres.go	75.90% <100.00%> (ø)
warehouse/utils/utils.go	81.74% <ø> (-0.27%)	⬇️
warehouse/integrations/redshift/redshift.go	73.85% <50.00%> (ø)
warehouse/integrations/tunnelling/connect.go	94.73% <89.28%> (ø)

... and 11 files with indirect coverage changes

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