Provide command to verify connection status

In GitLab by @jannismain on May 19, 2021, 15:33

Rationale

A ping-like command is required to test, whether

the connection has been established successfully (right after it has been established)
the server is ready to handle requests (also after longer periods of inactivity)
the connection is compromised in any other way not reflected by the TCP connection status (e.g. server busy)

This could also be used to check, whether the server is still alive, in cases where a server status is reported.

As a libjapi client
I want to check whether the connection to the server is working
In order to inform the user, when the connection might not be working properly

The messages could look like this:

Request:

{
    "japi_request": "japi_ping"
}

Response:

{
    "japi_response": "japi_ping",
    "data": {
        "success": true
    }
}