Add a `Computed` type decorator

[ecordell]

We often have the case where we have some object from the cache/cluster and some "computed" form of that same object (for the sake of comparing and determining whether changes need to be applied to the cluster).

In a few places we've made type aliases i.e. type ComputedFoo Foo so that it's clear at the type level where a given object is coming from. It might be nice to have a generic annotation, i.e foo = Computed[Foo] that we can use. We may be able to use this to enforce good practice (i.e. anything passed to a write should be a Computed type, etc).