### 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
this.namespace = List.of(NAMESPACE_VALUE_SOURCE);
        } else {
            this.namespace = namespace;
        }
        this.destination = destination;
    }
    @Override
    public IngestDocument execute(IngestDocument ingestDocument) throws Exception {
        if (destination != null) {
            ingestDocument.reroute(destination);
            return ingestDocument;
        }
        final String indexName = ingestDocument.getFieldValue(IngestDocument.Metadata.INDEX.getFieldName(), String.class);
        final String type;
        final String currentDataset;
        final String currentNamespace;
        
        int indexOfFirstDash = indexName.indexOf('-');
        if (indexOfFirstDash < 0) {
            throw new IllegalArgumentException(format(NAMING_SCHEME_ERROR_MESSAGE, indexName));
        }
        int indexOfSecondDash = indexName.indexOf('-', indexOfFirstDash + 1);
        if (indexOfSecondDash < 0) {
            throw new IllegalArgumentException(format(NAMING_SCHEME_ERROR_MESSAGE, indexName));
        }
        type = parseDataStreamType(indexName, indexOfFirstDash);
        currentDataset = parseDataStreamDataset(indexName, indexOfFirstDash, indexOfSecondDash);
        currentNamespace = parseDataStreamNamespace(indexName, indexOfSecondDash);
        String dataset = determineDataStreamField(ingestDocument, this.dataset, currentDataset);
        String namespace = determineDataStreamField(ingestDocument, this.namespace, currentNamespace);