[FEATURE](node-postgres): extending NodePgDatabase

### Feature hasn't been suggested before.

- [x] I have verified this feature I'm about to request hasn't been suggested before.

### Describe the enhancement you want to request

I'd like to build a superclass on top of NodePgDatabase but the builder function which returns it (called `construct`, i believe) seems not available publicly. the fact that the class is not buildable by itself makes the code structure not extendable, even tho, since it's a class, should be.

The symbol i'm talking about is:

which is used in:

it calls some (internal?) functions like `extractTablesRelationalConfig` and sets some private symbols `db.$xxx` so i can't really copy/paste the code.

any idea how to solve this?

Start by tracing NodePgDatabase and the construct builder, then inspect the internal extractTablesRelationalConfig call and private db.$xxx symbols mentioned in the issue. Done means a supported public extension path allows a superclass to build NodePgDatabase without copying internal implementation details.

Written by the indexing model from the issue text.