Add support sketch arguments.

Add support for generating sketch arguments. For instance, a sketch may operate on a range of objects, set up a way to for the user to define that range then have the sketch present them with all available options through the runner.

Start by locating the sketch runner and the entry points that generate or expose sketch options. Read how sketches currently receive inputs, then scope the range-selection flow described in the issue. Done means a sketch can define a range of objects and the runner presents all available options to the user.

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