Implement ping-pong for WebSocket server

[lexnv]

This PR exposes the ping-pong interface of the WebSocket server.

This is the counterpart of the client #772, requested by #738 .

Testing Done

• modified examples/ws into two binaries, one for the server, one for the client; started them with trace logs

2022-05-27T10:24:20.552095Z DEBUG jsonrpsee_ws_server::server: send ping
2022-05-27T10:24:20.552336Z TRACE soketto::connection: bd133ba0: send: (Ping (fin 1) (rsv 000) (mask (0 0)) (len 0))
2022-05-27T10:24:20.552676Z TRACE soketto::connection: bd133ba0: Sender flushing connection
2022-05-27T10:24:20.556269Z TRACE soketto: read 2 bytes
2022-05-27T10:24:20.556340Z TRACE soketto: read 4 bytes
2022-05-27T10:24:20.556374Z TRACE soketto::connection: bd133ba0: recv: (Pong (fin 1) (rsv 000) (mask (1 fba99e45)) (len 0))
2022-05-27T10:24:20.556414Z DEBUG jsonrpsee_ws_server::server: recv pong

[niklasad1]

LGTM, but would be good if you could replace the select!

[lexnv]

To have symmetry with the client implementation, added the following debug logs for Ping / Pong frames:

2022-05-27T10:24:20.552095Z DEBUG jsonrpsee_ws_server::server: send ping
...
2022-05-27T10:24:20.556414Z DEBUG jsonrpsee_ws_server::server: recv pong

[jsdw]

LGTM :)