Add GET /peer discoverability route

[mkurapov]

Context

In order to provide information that a new peer needs in order to peer with the Rafiki instance, we need to add a new route within the Rafiki backend. We can create a new "ilp peering" server within backend that exposes a GET /peer route. This route needs to return the

• ilpAddress
• list of supported assets
  of the Rafiki instance.

Todos

• Create a new ilp server that exposes GET /peer route and return the given information
• Put this new server instantiation under an ENABLE_AUTO_PEERING environment flag