Implement NextResultSet in a way compatible with sql.Rows.NextResultSet()

**Is your feature request related to a problem? Please describe.**
When trying to use pgx in a multi database module, like scany or similar, and trying to be compatible with database/sql it fails because NextResultSet like in sql.Rows is not implemented.
Also see https://github.com/jackc/pgx/issues/308 and https://github.com/jackc/pgx/issues/1512
Yes I know you "shouldn't" use NextResultSet and I really don't like it but I proprietary system I have to live with returns multiple result sets and that isn't something I can change.

**Describe the solution you'd like**
Even if it's slow and not optimal, perhaps it's possible to emulate it using MultiResultReader or something.
I'd gladly help but I do not have detailed enough knowledge about pgx to figure out if it's even possible.

**Describe alternatives you've considered**
Just implement the function and always return false for now.

**Additional context**

Start by comparing pgx's existing result-reading behavior with database/sql.Rows.NextResultSet(), then review the related issues #308 and #1512. Determine whether MultiResultReader can support compatible traversal of multiple result sets; done means callers can use the database/sql-compatible operation with the described proprietary system.

Written by the indexing model from the issue text.