### 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
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */
package org.elasticsearch.ingest.common;
import org.elasticsearch.core.Nullable;
import org.elasticsearch.ingest.AbstractProcessor;
import org.elasticsearch.ingest.ConfigurationUtils;
import org.elasticsearch.ingest.IngestDocument;
import org.elasticsearch.ingest.Processor;
import java.util.List;
import java.util.Locale;
import java.util.Map;
import java.util.Objects;
import java.util.function.Function;
import java.util.regex.Pattern;
import static org.elasticsearch.core.Strings.format;
import static org.elasticsearch.ingest.ConfigurationUtils.newConfigurationException;
import static org.elasticsearch.ingest.common.RerouteProcessor.DataStreamValueSource.DATASET_VALUE_SOURCE;
import static org.elasticsearch.ingest.common.RerouteProcessor.DataStreamValueSource.NAMESPACE_VALUE_SOURCE;
public final class RerouteProcessor extends AbstractProcessor {