[FEATURE] Add setting to enable/disable async query capability

[penghuo]

Is your feature request related to a problem?
I want to disable async_query capability to reject all coming requests.

What solution would you like?
A clear and concise description of what you want to happen.

sh$ curl -sS -H 'Content-Type: application/json' -X PUT localhost:9200/_cluster/settings \
... -d '{"transient":{"plugins.async_query.enable":"false"}}'
{
    "acknowledged": true,
    "persistent": {},
    "transient": {
        "plugins": {
            "async_query": { 
                "enable" :  false
            }
        }
    }
}

What alternatives have you considered?
n/a

Do you have any additional context?
Add any other context or screenshots about the feature request here.