This commit enhances the ClusterStatsNodes response to include global
processor usage stats on a per-processor basis.

example output:

```
...
    "processor_stats": {
      "gsub": {
        "count": 0,
        "failed": 0
        "current": 0
        "time_in_millis": 0
      },
      "script": {
        "count": 0,
        "failed": 0
        "current": 0,
        "time_in_millis": 0
      }
    }
...
```

The purpose for this enhancement is to make it easier to collect stats on how specific processors are being used across the cluster beyond the current per-node usage statistics that currently exist in node stats.

Closes elastic#46146.

The introduction of processor types into IngestStats had a bug.
It was set to `null` and set as the key to the map. This would
throw a NPE. This commit resolves this by setting all the processor
types from previous versions that are not serializing it out to
`_NOT_AVAILABLE`.