client: add command service connector stop

[jclarysse]

About this change: What it does, why it matters

Add client command for API endpoint ServiceKafkaConnectStopConnector

$ avn service connector stop --help                           
usage: avn service connector stop [-h] [--project PROJECT] service_name connector

Stop a Kafka connector

positional arguments:
  connector          Connector name
  service_name       Service name

options:
  -h, --help         show this help message and exit
  --project PROJECT  Project name to use, default None

[jclarysse]

Example scenario:

$ avn service connector status --project $PROJECT_NAME $SERVICE_NAME $CONNECTOR_NAME
{
    "status": {
        "state": "RUNNING",
        "tasks": []
    }
}
$ avn service connector stop --project $PROJECT_NAME $SERVICE_NAME $CONNECTOR_NAME
$ avn service connector status --project $PROJECT_NAME $SERVICE_NAME $CONNECTOR_NAME
{
    "status": {
        "state": "STOPPED",
        "tasks": []
    }
}

$ avn service connector resume --project $PROJECT_NAME $SERVICE_NAME $CONNECTOR_NAME
$ avn service connector status --project $PROJECT_NAME $SERVICE_NAME $CONNECTOR_NAME
{
    "status": {
        "state": "RUNNING",
        "tasks": []
    }
}