### 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/test/java/org/elasticsearch/ingest/common/RerouteProcessorTests.java
null
        );
    }
    private RerouteProcessor createRerouteProcessor(String destination) {
        return new RerouteProcessor(null, null, List.of(), List.of(), destination);
    }
    private void assertDataSetFields(IngestDocument ingestDocument, String type, String dataset, String namespace) {
        if (ingestDocument.hasField("data_stream")) {
            assertThat(ingestDocument.getFieldValue("data_stream.type", String.class), equalTo(type));
            assertThat(ingestDocument.getFieldValue("data_stream.dataset", String.class), equalTo(dataset));
            assertThat(ingestDocument.getFieldValue("data_stream.namespace", String.class), equalTo(namespace));
        } else {
            assertThat(ingestDocument.getCtxMap().get("data_stream.type"), equalTo(type));
            assertThat(ingestDocument.getCtxMap().get("data_stream.dataset"), equalTo(dataset));
            assertThat(ingestDocument.getCtxMap().get("data_stream.namespace"), equalTo(namespace));
        }
        assertThat(ingestDocument.getFieldValue("_index", String.class), equalTo(type + "-" + dataset + "-" + namespace));
        if (ingestDocument.hasField("event.dataset")) {
            assertThat(
                ingestDocument.getFieldValue("event.dataset", String.class),
                equalTo(ingestDocument.getFieldValue("data_stream.dataset", String.class))
            );
        }
    }
    private static IngestDocument createIngestDocument(String dataStream) {
        IngestDocument ingestDocument = RandomDocumentPicks.randomIngestDocument(random());
        ingestDocument.setFieldValue("_index", dataStream);
        return ingestDocument;
    }
    private static class SkipProcessor implements WrappingProcessor {