[Feature] Collections to agent keys

[matthme]

Currently, collections can only be made from an anchor to entry-types. I often have the case in my happs where I want to make collections of agent public keys, for example an ALL_AGENTS anchor.
It would be nice if that were supported as well. I would consider this a low-importance feature request though since it's not too hard to implement that on your own. The most costly part currently is to write the validation boilerplate for the associated link types (since scaffolding link types pointing to agent keys is currently not supported by the scaffolding).