feat: add delegate routers to libp2p config

[jacobheun]

Adds the delegate routing modules needed for #2155.

This enables js-ipfs nodes to leverage external nodes to provide content, find content, and find peers.

I added a crude randomizer for the preload nodes which are supporting delegate routing. Ideally this would be per request, but the modules will need to be updated to take multiple hosts to do so.

A note: If the DHT is enabled on your node, libp2p will attempt to perform the action using the DHT first. If that is not successful, it will then use the delegates to perform the request. This gives nodes the ability to fallback to the external nodes while they are building up their DHT records/peers.

[dirkmc]

Should the hostnames be in config?
Otherwise LGTM 👍

[jacobheun]

Should the hostnames be in config?

Yeah, we should probably have the multiaddrs in there and then pull out the http endpoints.

This will cause the config to deviate from go, as we'll likely add a new address field like Addresses.Delegates. This should be fine as it's more of a js thing, but if we do that we should check to make sure go doesn't care about the extra field.

Does Addresses.Delegates sound like the right place for this to go?

Addresses: {
  ...
  Delegates: [
    '/dns4/node0.delegate.ipfs.io/tcp/443/https',
    '/dns4/node1.delegate.ipfs.io/tcp/443/https'
  ]
}

(updated addrs to match ones from #2291 – @lidel)

[dirkmc]

Discovery and Bootstrap are at the top level, so it may make sense for Delegates to be as well, and thereby less likely to conflict with go config

[jacobheun]

I've updated this to only add delegated routing if the delegates multiaddrs are actually added to the Config Addresses.Delegates property. I also added docs and a couple tests.

[alanshaw]

Sorry just noticed the link to the example is not working when rendered.

Separately the example says this depends on a branch of go ipfs - does it still?

Also, we need to document in the example what API endpoints need to be exposed on a go ipfs node for this to work.

[alanshaw]

@jacobheun I fixed the link and rebased