Dependencies between graph-backed assets without input dependencies

[dpeng817]

Currently, software-defined assets declare their dependencies via inputs to the underlying node. Sometimes, it's advantageous to declare dependencies without having the dependency correspond to an input. In cases such as these, the dependencies would be represented purely by asset key.

Relevant discussion on how to have a subgraph depend on another subgraph: https://github.com/dagster-io/dagster/discussions/9930.