Add support for operation hedging, as in SERVER-45432, with an explicit readPreference.

The form of that readpreference should be:

// some existing command
{
  ...,
  $readPreference: {
    mode: "nearest",
    hedge: {
      enabled: true,   // default value is true
    }
}

which should map to RemoteCommandRequestOnAny::HedgeOptions of

• count = 1
• as in implicit readPreference nearest