Ergonomic way to define enums for use by nodes

[Keavon]

Right now it's a laborious process to add an enum to the TaggedValue. We need to auto-generate ways of turning an enum into a dropdown menu or radio button input widget. And we need to consider how users will be able to define their own once they can write Rust code to create custom nodes.

I discussed some possible approaches with @TrueDoctor a couple days ago. More thought will be required once we can begin tackling this.

[kythyria]

Resolved by #2589.