[lib] Make registerConfig take full Config instead of $Shape<Config>

Summary:
Following the Flow upgrade, I'm looking at replacing all usages of $Shape in our codebase. Context here.

When I swapped this particular $Shape into a Partial, it revealed a type error. We can't set registeredConfig, which is a ?Config, to a Partial<Config>.

Instead I just updated the type to take a Config, which works with both of its callsites anyways.

Test Plan: Flow

Reviewers: michal

Reviewed By: michal

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10082