Enable resources to "require" other resources

[hugosantos]

At the moment providers can declare resources explicitly that they consume. But sometimes users may want to attach dependencies to some generic resources, like kubernetes manifests.

This could work similarly to "requires" in servers, that simply establishes a link between two servers.

Exact syntax + semantics TBD. Probably the provider should either (a) specify a filter for resources, or (b) declare that the resource class allows any dependency.