Allow overriding root instance of partition

[LPGhatguy]

In order to support syncing directly into services or into other interesting instance types, it'd be useful to specify the name, class, and properties of a partition root in addition to a path to sync from.

I imagine it would look something like this in a project:

"ReplicatedStorage": {
	"$className": "ReplicatedStorage",
	"$path": "src/replicated-storage"
}

The extra metadata available to configure project nodes will be relevant here most likely, like $ignoreUnknownInstances.