libp2p retrieval transports endpoint

[dirkmc]

Fixes #717

TODO:

• Change Protocol Endpoint field to an array of multi-addresses
• Implement CLI command to make libp2p query to endpoint

$ boost provider retrieval-transports t01000
libp2p
  /ip4/192.168.1.11/tcp/50000
  /ip4/127.0.0.1/tcp/50000
  /ip6/::1/tcp/65450
http
  /dns/foo.com/tcp/443/https
    https://foo.com:443

$ boost --json provider retrieval-transports t01000
{
  "http": [
    {
      "multiaddr": "/dns/foo.com/tcp/443/https",
      "address": "https://foo.com:443"
    }
  ],
  "libp2p": [
    {
      "multiaddr": "/ip4/192.168.1.11/tcp/50000"
    },
    {
      "multiaddr": "/ip4/127.0.0.1/tcp/50000"
    },
    {
      "multiaddr": "/ip6/::1/tcp/65450"
    }
  ]
}

[ribasushi]

This looks excellent. Left a single comment regarding a... comment.

[ribasushi]

@dirkmc @hannahhoward where does libp2p-http fit in this scenario?

[dirkmc]

Currently libp2p-http is only used for storage deals