feat(shared-data): allow default version finding for labware schema v3

[caila-marashaj]

Overview

Currently, in order to load in a schema v3 labware definition, you need to load the labware with the version number that's listed in schema 3. Otherwise, you'll get the schema 2 definition. This pr maintains that functionality, but adds the ability to find a default schema 3 definition for any applicable labware by loading in the labware like this:
load_labware(load_name=..., location=..., schema=3)

Its not much of a change, but it's a little easier than going and finding the appropriate version number in shared-data, and might come in handy when we eventually do want to load in labware definitions in schema 3 by default.