arti-rpc-client-core: Make from_connect_string() a JSON string

In arti-rpc-client-core, I propose making from_connect_string() a JSON string. So far, the format is:

{
    "protocol": "unix",
    "location": "/tmp/arti.sock",
}

I don't know if this is the desired format or if it should be another way, or if the value names should be changed.