[Feature] Link-types from AnyLinkableHash to AnyLinkableHash

[matthme]

Currently hc scaffold link-type only allows to create links from entry types to entry types, each of which must already have been created. A link in general however can point from an AnyLinkableHash to an AnyLinkableHash. A common use case for me is for example to make a link to or from an AgentPublicKey, I would like to have that be supported.