[DOP-23742] Fix writing to Postgres and Greenplum behind PGBouncer

[dolfinus]

Change Summary

Depends on #337.

Fixed issue with writing data to PG/GP instances behind pgbouncer with pool_mode=transaction, caused by pgjdbc/pgjdbc#848. Added PGBouncer to docker-compose and CI workflows, running tests on PG/GP with and without PGBouncer.

Behavior is almost the same as before, except rare case of Postgres/Greenplum calling connection.fetch("SELECT function()") where function calls DML/DDL - previously this failed due to read-only connection, now it's not. This was sacrificed for pgbounce support.

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.02%. Comparing base (fef4ecc) to head (6fb3cf0).
Report is 69 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #336      +/-   ##
===========================================
+ Coverage    91.89%   92.02%   +0.12%     
===========================================
  Files          228      228              
  Lines         9836     9840       +4     
  Branches      1008     1008              
===========================================
+ Hits          9039     9055      +16     
+ Misses         605      598       -7     
+ Partials       192      187       -5     

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