feat: connection_string config property

[elephantum]

use case: heroku postgres

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Proposed changes

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Checklist

• Description above provides context of the change
• I have added tests that prove my fix is effective or that my feature works
• Unit tests for changes (not needed for documentation changes)
• CI checks pass with my changes
• Bumping version in setup.py is an individual PR and not mixed with feature or bugfix PRs
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• Relevant documentation is updated including usage instructions