### Elasticsearch Version

main

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

n/a

### Problem Description

This requires further verification, but I suspect comma-separated string of source index name in transform configuration:
```
"source": {
  "index": "my-index*,my-other-index*"
}
```

may not resolve properly and needs to be replaced with:
```
"source": {
  "index": ["my-index*","my-other-index*"]
}
```

We should make both syntaxes work equally fine.

### Steps to Reproduce

Provide comma-separated list of indices in the `"source"` section of transform config:
```
"source": {
  "index": "my-index*,my-other-index*"
}
```

### Logs (if relevant)

_No response_
x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/transform/transforms/SourceConfig.java
import org.elasticsearch.common.io.stream.StreamOutput;
import org.elasticsearch.common.io.stream.Writeable;
import org.elasticsearch.license.RemoteClusterLicenseChecker;
import org.elasticsearch.search.builder.SearchSourceBuilder;
import org.elasticsearch.xcontent.ConstructingObjectParser;
import org.elasticsearch.xcontent.NamedXContentRegistry;
import org.elasticsearch.xcontent.ParseField;
import org.elasticsearch.xcontent.ToXContentObject;
import org.elasticsearch.xcontent.XContentBuilder;
import org.elasticsearch.xcontent.XContentParser;
import org.elasticsearch.xpack.core.deprecation.DeprecationIssue;
import org.elasticsearch.xpack.core.transform.TransformField;
import org.elasticsearch.xpack.core.transform.utils.ExceptionsHelper;
import java.io.IOException;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.function.Consumer;
import static java.util.stream.Collectors.toMap;
import static org.elasticsearch.xcontent.ConstructingObjectParser.constructorArg;
import static org.elasticsearch.xcontent.ConstructingObjectParser.optionalConstructorArg;
public class SourceConfig implements Writeable, ToXContentObject {
    public static final ParseField QUERY = new ParseField("query");
    public static final ParseField INDEX = new ParseField("index");
    public static final ConstructingObjectParser<SourceConfig, Void> STRICT_PARSER = createParser(false);
    public static final ConstructingObjectParser<SourceConfig, Void> LENIENT_PARSER = createParser(true);
    @SuppressWarnings("unchecked")
    private static ConstructingObjectParser<SourceConfig, Void> createParser(boolean lenient) {