Add a way for package maintainers to specify their own overrides

In developing Socket Registry I have been noodling on giving package maintainers the ability to specify their own overrides. So we can include them where it makes sense (in our registry/manifest.json). \cc @ljharb

I'd like the data to be accessible via the npm registry. We query the registry to create the manifest per package looking for things like "deprecated". Let's keep it simple. If a version is released with a prerelease of "override" we can defer to it for some category of overrides, like "cleanup".