Support constant value sources on connections

[mtrberzi]

This may also be an issue for core but I'm putting it here because it is relevant to frontend code generation as well. Right now the only way to (potentially) get a constant value to be the source end of a connection is to define a new node type whose only job is to hold that constant value. Unfortunately, the frontend needs to know about this, which might be domain-specific. It would be good to generalize the idea of a "constant driver" in a domain-agnostic way, and then just add support in backends for this special node type.