This is the layer that keeps track of the different groups of hooks and
can filter them before runtime. The idea here is that you'll be able
to do things like skip hooks that require network access when you don't
have it, or skip CPU-intensive hooks when you don't need them, etc.
This is the simplest version of a common validation we're going to do:
make sure that a particular piece of metadata has one of a set of
values.
This checker needs some bounds checking but I wanted to err on the
side of committing this early because it introduces so much base
infrastructure.
