Additional SSL configuration for database

Right now, the Backend database config provides limited support around SSL. The config may include an optional `ssl` property, which if specified, must be `true`. When `ssl` is `true`, `rejectUnauthorized` is automatically specified as `false`. We set it up this way because Slonik provided limited configuration around SSL and seemed to set `rejectUnauthorized` to `false`. We also wanted to ensure that Slonik and Knex connect to the database in the same way. (For additional background, see getodk/central-backend#377 and getodk/central-backend#394.)

That said, the latest release of Slonik (v25.1.0) provides additional options around SSL: see gajus/slonik#159 and gajus/slonik@6c3b178c9b52418b348fb0d8a7a8e75fb31047f1. It looks like it's now possible to use SSL without setting `rejectUnauthorized` to `false`.

I think this is a useful change, but I also think it might be a breaking change for us. Slonink seems to no longer use `pg-connection-string` to parse the connection string. Relatedly, it seems to no longer support `ssl` as a query parameter: it wants SSL options to be specified in a different way.

I think we'll need to look more into this before upgrading to v25.1.0 or later. It's also nice that this would provide a way for us to support additional SSL configuration.

CC also @florianm, who I know is interested in these options.