It's currently not possible connect to Postgres with custom certificates. It is something we have been working on implementing, so certificate handling was available for all the database types we support, but it turned out to be a lot more complicated than we initially thought. I currently don't have a timeline for this.
@yasaichi We are working on implementing more connection handling in the app. Still unsure if it will be finished in time for v41 or if it will be v42. It's a lot more complicated, when it has to support 15+ different database types across multiple versions.