### 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
public Map<String, Object> getScriptBasedRuntimeMappings() {
        return getRuntimeMappings().entrySet()
            .stream()
            .filter(e -> e.getValue() instanceof Map<?, ?> && ((Map<?, ?>) e.getValue()).containsKey("script"))
            .collect(toMap(Map.Entry::getKey, Map.Entry::getValue));
    }
    public ActionRequestValidationException validate(ActionRequestValidationException validationException) {
        return queryConfig.validate(validationException);
    }
    public void checkForDeprecations(String id, NamedXContentRegistry namedXContentRegistry, Consumer<DeprecationIssue> onDeprecation) {
        queryConfig.checkForDeprecations(id, namedXContentRegistry, onDeprecation);
    }
    public boolean requiresRemoteCluster() {
        return Arrays.stream(index).anyMatch(RemoteClusterLicenseChecker::isRemoteIndex);
    }
    @Override
    public void writeTo(StreamOutput out) throws IOException {
        out.writeStringArray(index);
        queryConfig.writeTo(out);
        out.writeGenericMap(runtimeMappings);
    }
    @Override
    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
        builder.startObject();
        builder.array(INDEX.getPreferredName(), index);
        if (params.paramAsBoolean(TransformField.EXCLUDE_GENERATED, false) == false) {
            builder.field(QUERY.getPreferredName(), queryConfig);
        } else if (queryConfig.equals(QueryConfig.matchAll()) == false) {
            builder.field(QUERY.getPreferredName(), queryConfig);
        }