feat: support knex

[olavloite]

Adds a small sample application for using Knex with PGAdapter
and Cloud Spanner. Knex is a query builder built on top of the
standard Node.js 'pg' driver, which is already supported. This
means that Knex.js indirectly is also supported with PGAdapter.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (542f2dc) 90.66% compared to head (69e65ae) 90.66%.

Additional details and impacted files

@@                  Coverage Diff                  @@
##             postgresql-dialect    #1169   +/-   ##
=====================================================
  Coverage                 90.66%   90.66%           
  Complexity                 2640     2640           
=====================================================
  Files                       141      141           
  Lines                      9016     9016           
  Branches                   1313     1313           
=====================================================
  Hits                       8174     8174           
  Misses                      567      567           
  Partials                    275      275           

Flag	Coverage Δ
all_tests	90.66% <ø> (ø)

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