Add ping protocol to libp2p stack

[flcl42]

Protocol implementation request: ping

• Stack: standard libp2p
• Type: application layer
• Specification: https://github.com/libp2p/specs/blob/master/ping/ping.md
• See also: https://docs.libp2p.io/concepts/introduction/protocols/ping/

Additional context Ping is needed for interop tests for one time communication. Pings ability to do a from time to time lifelines check seems not yet covered by core API and is a reason for suggestions and discussion.