### 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
private String determineDataStreamField(
        IngestDocument ingestDocument,
        List<DataStreamValueSource> valueSources,
        String fallbackFromCurrentTarget
    ) {
        
        
        for (DataStreamValueSource value : valueSources) {
            String result = value.resolve(ingestDocument);
            if (result != null) {
                return result;
            }
        }
        
        return fallbackFromCurrentTarget;
    }
    @Override
    public String getType() {
        return TYPE;
    }
    List<DataStreamValueSource> getDataStreamDataset() {
        return dataset;
    }
    List<DataStreamValueSource> getDataStreamNamespace() {
        return namespace;
    }
    String getDestination() {
        return destination;
    }
    public static final class Factory implements Processor.Factory {