### 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
* @param runtimeMappings Search-time runtime fields that can be used by the transform
     */
    public SourceConfig(String[] index, QueryConfig queryConfig, Map<String, Object> runtimeMappings) {
        ExceptionsHelper.requireNonNull(index, INDEX.getPreferredName());
        if (index.length == 0) {
            throw new IllegalArgumentException("must specify at least one index");
        }
        if (Arrays.stream(index).anyMatch(Strings::isNullOrEmpty)) {
            throw new IllegalArgumentException("all indices need to be non-null and non-empty");
        }
        this.index = index;
        this.queryConfig = ExceptionsHelper.requireNonNull(queryConfig, QUERY.getPreferredName());
        this.runtimeMappings = Collections.unmodifiableMap(
            ExceptionsHelper.requireNonNull(runtimeMappings, SearchSourceBuilder.RUNTIME_MAPPINGS_FIELD.getPreferredName())
        );
    }
    public SourceConfig(final StreamInput in) throws IOException {
        index = in.readStringArray();
        queryConfig = new QueryConfig(in);
        runtimeMappings = in.readMap();
    }
    public String[] getIndex() {
        return index;
    }
    public QueryConfig getQueryConfig() {
        return queryConfig;
    }
    public Map<String, Object> getRuntimeMappings() {
        return runtimeMappings;
    }