Serving agent activity for updated and previous agent keys

[jost-s]

When an agent updates their key, agent authority will change to another peer. In turn an agent authority can only serve the segment of the source chain they're an authority for.

The coordination layer needs a way to request different segments of the complete source chain, of any key of the agent's key lineage.

The previous agent's key can be fetched by getting the previous action of the first action of the segment. Its author is the previous agent key that must be returned.

The next agent's key is in the entry of the last action of the current segment, which must be an Update action of the agent key.

AC:

• call returns segment of the chain's agent activity for the current agent key
• get_agent_activity returns a "previous_agent" field, and maybe a "next_agent" field, forming a doubly-linked-list of chain segments that can be traversed
• Make clear in docs that only the segment of the current agent key is returned. Further segments of the chain by previous agent keys need to be queried explicitly.

[jost-s]

Convenience feature. Lowering priority because apps can achieve the same without this feature.

[maackle]

The workaround is that HDK would query the new chain to find the OpenChain action to get the prev cell ID, and CloseChain in the prev chain to get the next cell ID