Explicit input instantiation escape hatch

**Is your feature request related to a problem? Please describe.**

I'm designing a stable external API that needs to wrap our existing fast-moving internal API. Sometimes, I want to call existing resolvers to do that.

However, when using deeply nested inputs with a `@property`, it's not possible to wrap that input with another (simplified) input, because I can't create an "instance" with the correct parameters on creation.

**Describe the solution you'd like**
I would like an escape hatch to instantiate inputs.

**Describe alternatives you've considered**
An inheritance-based mechanism where I could remove fields and structures from a parent might solve the problem, but I don't think it would be as clean.