feat: implement .delegate on capabilities

[Gozala]

This adds .delegate method to the capability defs which:

1. Validates with and provided nb fields.
2. Unlike invoke / create it allows omitting nb fields, because in delegation all the nbs are optional.
3. It does not currently does proof validation because that would require PrincipalParser and capability loaders to be provided. I think we'll be able to add that in the future once we have Agent abstraction that will be passed into issuer field.

All in all it improves things by:

1. Removing a need to call .delegate() after .invoke(...).
2. Removes a need for marking nb fields optional in capabilty def.

Note this is based on #109 and targets it as opposed to main branch to reduce need for doing conflict resolution.