introspection support: implement host.IntrospectableHost and wire up the introspector

[aarshkshah1992]

For #775.

This PR introduces changes to the Runtime message and integrations tests for the Introspector for the introspection 2.0 protocol defined at libp2p/go-libp2p-core#142.

To try it out :

h1, err := New(ctx,
		Introspector(
			introspector.NewDefaultIntrospector(),
			introspector.WsServerWithConfig(&introspector.WsServerConfig{
				ListenAddrs: []string{iaddr},
			}),
		),
		BandwidthReporter(metrics.NewBandwidthCounter()),
	)

[raulk]

Closing this PR and opening a new one to describe the changeset more accurately.

[raulk]

Superseded by #964.