### Elasticsearch Version

8.8.1

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

ESS

### Problem Description

The `reroute` processor allows to specific Mustache like syntax for field references. In contrast to other processors (.e.g `set` or `rename`) the variable does not allow to include spaces between the field name and the curly brackets.

This doesn't work:
```
"dataset": "{{ _reroute.dataset }}"
```
while this does:
```
"dataset": "{{_reroute.dataset}}"
```


### Steps to Reproduce

```
PUT _ingest/pipeline/reroute_example
{
  "processors": [
    {
      "reroute": {
        "tag": "example",
        "dataset": "{{ _reroute.dataset }}"
      }
    }
  ]
}

POST logs-test.original-default/_doc?pipeline=reroute_example
{
  "_reroute": {
    "dataset": "test.reroute"
  },
  "@timestamp": "2023-06-20"
}
```

Notice how the target Data Stream is `logs-test.original-default` instead of `logs-test.reroute-default`

### Logs (if relevant)

_No response_
modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/RerouteProcessor.java
String value = ingestDocument.getFieldValue(fieldReference, String.class, true);
                if (value == null) {
                    value = getStringFieldValueInDottedNotation(ingestDocument);
                }
                return sanitizer.apply(value);
            } else {
                return value;
            }
        }
        private String getStringFieldValueInDottedNotation(IngestDocument ingestDocument) {
            String value = null;
            Object valueObject = ingestDocument.getCtxMap().get(fieldReference);
            if (valueObject instanceof String) {
                value = (String) valueObject;
            } else if (valueObject != null) {
                throw new IllegalArgumentException(
                    "field ["
                        + fieldReference
                        + "] of type ["
                        + valueObject.getClass().getName()
                        + "] cannot be cast to ["
                        + String.class.getName()
                        + "]"
                );
            }
            return value;
        }
    }
}