Select specific host when running debug command

[lthibault]

Currently, ww debug's subcommands target an arbitrary host, the one that with which the client happens to be connected. The debug commands take a <peer> argument, intended to be either an instance ID or a libp2p peer.ID. This should be used to designate the node being debugged.

The preferred way to achieve this is through the Anchor API, which still needs to be re-implemented after the major overhaul of the cluster.Host capability. As before, we want to define a Go interface for Anchor so that we can present a type-switch based API for interacting with various anchor types (cluster, host, in-host path...). Asking users to recast capability clients is bound to confuse them.

Note that only a partial implementation of anchor is needed, as only root and host anchors are required.

Marking this as a bug since the current CLI's behavior is covert, and basically just wrong.

Tasks

Most of these are subtasks of #57.

• Define Go interface for Anchor
• Implement Anchor API methods in client.Node
• Implement anchor.Host; ensure it provides access to a Host capability
• Call Walk from ww debug, type-assert for anchor.Host, and obtain debugger