Support custom input types

[ianhoffman]

Allow registering custom input types. This seems really useful from a security standpoint: when coercing input to a Hack type, the GraphQL type itself can verify that the viewer has access to the resource using global context.

I haven't done any work to make defining custom types ergonomic. You have to subclass NamedType and then use TNamedInputType and TNonNullableType. We could definitely make this nicer.

I'm also not sure this works as-is with clients. For example, ChannelID is a new scalar type which takes a string and outputs a Channel object. Will clients understand that when they see ChannelID, they should provide a string?

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