Not setting the full dialect and driver for CONFIG_DATABASE_URI in case of an external DB currently breaks the docker image

[sdsys-ch]

When using PostgreSQL for pgAdmin's config database, SQLAlchemy 2.0+ requires the 'psycopg' dialect vs. 'psycopg2' or 'psycopg3':

>>> from sqlalchemy.dialects.postgresql import psycopg; help(psycopg)
Help on module sqlalchemy.dialects.postgresql.psycopg in sqlalchemy.dialects.postgresql:

NAME
    sqlalchemy.dialects.postgresql.psycopg

DESCRIPTION
    .. dialect:: postgresql+psycopg
        :name: psycopg (a.k.a. psycopg 3)
        :dbapi: psycopg
        :connectstring: postgresql+psycopg://user:password@host:port/dbname[?key=value&key=value...]
        :url: https://pypi.org/project/psycopg/

    ``psycopg`` is the package and module name for version 3 of the ``psycopg``
    database driver, formerly known as ``psycopg2``.  This driver is different
    enough from its ``psycopg2`` predecessor that SQLAlchemy supports it
    via a totally separate dialect; support for ``psycopg2`` is expected to remain
    for as long as that package continues to function for modern Python versions,
    and also remains the default dialect for the ``postgresql://`` dialect
    series.

Problem:

When setting PGADMIN_CONFIG_CONFIG_DATABASE_URI in Docker env to either postgresql://... or postgresql+psycopg3://... we either hit sqlalchemy.exc.NoSuchModuleError: Can't load plugin: sqlalchemy.dialects:postgresql.psycopg3 or ModuleNotFoundError: No module named 'psycopg2'.

This PR ensures proper DBAPI postgresql dialect handling for both initial container validation (via entrypoint) and runtime connections for an external postgres config database.

I chose this route because

1. Runtime database connections work with all URI formats through Flask-SQLAlchemy
2. No impact on pgadmin's internal postgres connections via wrapper function

Let me know if you prefer a different approach. Might have overlooked something / duplicate code isn't nice. Thanks.

[adityatoshniwal]

Otherwise, changes looks good to me.