…lastic#75606)

This PR makes the delayed allocation infrastructure aware of registered node shutdowns, so that reallocation of shards will be further delayed for nodes which are known to be restarting.

To make this more configurable, the Node Shutdown APIs now support a `allocation_delay` parameter, which defaults to 5 minutes. For example:
```
PUT /_nodes/USpTGYaBSIKbgSUJR2Z9lg/shutdown
{
  "type": "restart",
  "reason": "Demonstrating how the node shutdown API works",
  "allocation_delay": "20m"
}
```

Will cause reallocation of shards assigned to that node to another node to be delayed by 20 minutes. Note that this delay will only be used if it's *longer* than the index-level allocation delay, set via `index.unassigned.node_left.delayed_timeout`.

The `allocation_delay` parameter is only valid for `restart`-type shutdown registrations, and the request will be rejected if it's used with another shutdown type.

… transport client