Naive attempt to add support for dsn url config style for production db

This should allow to write your config file like:

web.yaml:

production_db: name: postgresql:///?service=swh-web

thus allowing to use pg_services and pgpass to declare postgresl access configuration.

Might help for containerized execution environment where db configuration can be dynamic.

Depends on !637 (closed)

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Migrated from D6336 (view on Phabricator)