### 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
public static final String TYPE = "reroute";
    private static final String NAMING_SCHEME_ERROR_MESSAGE =
        "invalid data stream name: [%s]; must follow naming scheme <type>-<dataset>-<namespace>";
    private static final String DATA_STREAM_PREFIX = "data_stream.";
    private static final String DATA_STREAM_TYPE = DATA_STREAM_PREFIX + "type";
    private static final String DATA_STREAM_DATASET = DATA_STREAM_PREFIX + "dataset";
    private static final String DATA_STREAM_NAMESPACE = DATA_STREAM_PREFIX + "namespace";
    private static final String EVENT_DATASET = "event.dataset";
    private final List<DataStreamValueSource> dataset;
    private final List<DataStreamValueSource> namespace;
    private final String destination;
    RerouteProcessor(
        String tag,
        String description,
        List<DataStreamValueSource> dataset,
        List<DataStreamValueSource> namespace,
        String destination
    ) {
        super(tag, description);
        if (dataset.isEmpty()) {
            this.dataset = List.of(DATASET_VALUE_SOURCE);
        } else {
            this.dataset = dataset;
        }
        if (namespace.isEmpty()) {