Adds script cache stats to `_node/stats`.
If using the general cache:
```
      "script_cache": {
        "sum": {
          "compilations": 12,
          "cache_evictions": 9,
          "compilation_limit_triggered": 5
        }
      }

```
If using context caches:
```
      "script_cache": {
        "sum": {
          "compilations": 13,
          "cache_evictions": 9,
          "compilation_limit_triggered": 5
        },
        "contexts": [
          {
            "context": "aggregation_selector",
            "compilations": 8,
            "cache_evictions": 6,
            "compilation_limit_triggered": 3
          },
          {
            "context": "aggs",
            "compilations": 5,
            "cache_evictions": 3,
            "compilation_limit_triggered": 2
          },
```
Refs: elastic#50152