add command to give basic information on a pool

[bmc-msft]

The command onefuzz status pool pool-name now provides something akin to:

❯ onefuzz status pool default
{
    "node_state": {
        "free": 5,
        "busy": 1
    },
    "tasks": {
        "312c725b-a3ca-4718-97b0-793b9a5ca27a": 1
    }
}