Stabilize how required resources (i.e. normalizers) are constructed

[jsstevenson]

It'd be good to avoid hard-coding too much, and let those resources manage the responsibility of checking env vars, etc. We should allow a constructed normalizer query handler to be passed through if it's already been declared, but otherwise, we ideally should be hands-off about how params are managed and passed.

Because there are some shared resources (e.g. Variation uses Gene) we need to make sure that this can be done in a composable manner, which could be tricky.