Add typing to dbcore types modules

[Serene-Arc]

Description

This is part of #4484, adding typings to the dbcore submodule. It's very much still a work in progress. I've made a couple of # TYPING ERROR comments where there are obvious things that don't match up with what the typings would seem to suggest. MyPy gives a lot more errors than my IDE though so there's clearly some work to be done.