Ask HN: Open-source tool to connect to customers' database?

I'm thinking of building an app that would require the user (developer) to connect their database to my app. A wide selection of options would be nice, but happy to just start with Postgres.

Are there open source tools that can facilitate this? I see a lot of apps have this capability but seems pretty complex so I assume not all of them are building this from scratch (storing credentials, IP whitelisting, retries, refreshing data, etc.)

Even SaaS products would be nice too. It just seems like a huge undertaking and I'd rather not try and build it from scratch.