Support dynamic result-shapes as well as dynamic sources

[Talden]

The tests demonstrate selection of a compile-time known shape (eg (i32, String) from a source unknown at compile-time.

However in a more dynamic scenario, the shape of the selection we want to make is unknown at compile-time but computable at runtime.

eg. A user is able to define customer columns for an entity. DDL is executed to adjust the table and customer column metadata is stored. Now efficient queries can be built against that table for which the number and order of columns is only known at runtime.

Naturally this is NOT what you want to do for a schema general-case, but is a common requirement for many systems - eg almost all CRM systems, HR systems, issue-tracking solutions such as JIRA and others.

Perhaps it should be possible to select into per-row hashmaps of column/value pairs where the value is an enum with database-type variants and database NULLs are omitted from the map.