Add pldebugger to dev db

[dmos62]

Indirectly part of my work on dynamic defaults.

Adds a debugger to our dev Postgres db. Helps to work with our plpgsql code that @mathemancer has been steadily introducing as part of the removing sqlalchemy project.

My current prefered way to use it is via pgadmin4's debugger. Before using, debugger's extension must be created via CREATE EXTENSION pldbgapi;.

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[mathemancer]

Looks good, seems not to break anything

[silentninja]

This PR is causing a regression. The dev-db service fails with the following error even after rebuilding the Docker image

 [1] FATAL:  could not access file "plugin_debugger": No such file or directory

Update: I rebuilt the image and I am not getting that error anymore.