### 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
ConstructingObjectParser<SourceConfig, Void> parser = new ConstructingObjectParser<>("data_frame_config_source", lenient, args -> {
            String[] index = ((List<String>) args[0]).toArray(new String[0]);
            
            QueryConfig queryConfig = args[1] == null ? QueryConfig.matchAll() : (QueryConfig) args[1];
            Map<String, Object> runtimeMappings = args[2] == null ? Collections.emptyMap() : (Map<String, Object>) args[2];
            return new SourceConfig(index, queryConfig, runtimeMappings);
        });
        parser.declareStringArray(constructorArg(), INDEX);
        parser.declareObject(optionalConstructorArg(), (p, c) -> QueryConfig.fromXContent(p, lenient), QUERY);
        parser.declareObject(optionalConstructorArg(), (p, c) -> p.map(), SearchSourceBuilder.RUNTIME_MAPPINGS_FIELD);
        return parser;
    }
    private final String[] index;
    private final QueryConfig queryConfig;
    private final Map<String, Object> runtimeMappings;
    /**
     * Create a new SourceConfig for the provided indices.
     *
     * {@link QueryConfig} defaults to a MatchAll query.
     *
     * @param index Any number of indices. At least one non-null, non-empty, index should be provided
     */
    public SourceConfig(String... index) {
        this(index, QueryConfig.matchAll(), Collections.emptyMap());
    }
    /**
     * Create a new SourceConfig for the provided indices, from which data is gathered with the provided {@link QueryConfig}
     *
     * @param index Any number of indices. At least one non-null, non-empty, index should be provided
     * @param queryConfig A QueryConfig object that contains the desired query, needs to be non-null