refactor: De-deprecate AgentData's addSpace()

[Peeja]

This was deprecated in #433, but it's still what we use, and no clear migration path was given.

@alanshaw Do you remember what the plan was? Should we do this and reverse it, or is there a better way we should write down?

[alanshaw]

Yeah, not fully remembered (sorry) but I think the idea was to signal early that we wanted to move away from the client storing state like the list of spaces, or the "current" space, beyond the delegations it has received/created. i.e. the list of spaces should be derivable from the delegations in the store.

My apologies - I should have documented and probably done in a separate PR...but hey, younger me was a maverick.

I'm happy to merge this but I still desire the goal above and so addSpace should not really exist, and addDelegation should be the only method here for adding spaces.