Fix WS clientConfig types and clarify docs

[cgewecke]

In #1354 @naddison36 reports that the types and documentation for part of the WebsocketProvider client config that enables a long running connection to Infura are incorrect.

copied comment

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For future devs looking for a solution to keeping a WebSocket connection to Infura alive, this worked for me

const provider = new Web3.providers.WebsocketProvider(
            config.PROVIDER_WS,
            {
                // @ts-ignore
                clientConfig: {
                    keepalive: true,
                    keepaliveInterval: 60000
                }
            }
        );

@ts-ignore is required for TypeScript as the Web3js typings is expecting a string for the clientConfig option which is wrong. The doco for the WebSocketClient used by Web3js also does not properly cover the keepalive config. https://github.com/theturtle32/WebSocket-Node/blob/master/docs/WebSocketClient.md#client-config-options

The WebSocketConnection code with the keepalive config is at https://github.com/theturtle32/WebSocket-Node/blob/574b703ca978709832c6c2f67d6c1de376d432b5/lib/WebSocketConnection.js#L111