Libp2p proxies

[ravenac95]

We need to support libp2p proxies so that we can better DoS protect networks. The proxies need to be able to do access control filtering similar to #1871.

Details

We would like to support an architecture that would look something like this for grpc and libp2p proxies:

Acceptance Criteria

• An interface is designed to properly expose the required access control policy to support a LibP2P proxy
  • It is possible that this can use the exact same interface as Support access control policy filtering on GRPC Proxies #1829
• Either a general purpose proxy or Ekiden specific proxy is created for Ekiden's Libp2p communications
• The Ekiden-Node can be configured to accept proxied requests over libp2p.