[Proposal] Row Constructors

This feature allows the model configuration to define a custom row generation method, which is called whenever a row is pulled from the database. The current `create` system is fine for most scenarios, but there are instances where having some control over what the returned row result looks like is desirable. Rather than have to hack this into the stream, it makes sense for `streamsql` to be able to handle this accordingly.

~~I'm not 100% convinced about naming, so feel free to make suggestions.~~