### 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/test/java/org/elasticsearch/xpack/core/transform/transforms/SourceConfigTests.java
}
    @Override
    protected SourceConfig doParseInstance(XContentParser parser) throws IOException {
        return SourceConfig.fromXContent(parser, lenient);
    }
    @Override
    protected SourceConfig createTestInstance() {
        return lenient ? randomBoolean() ? randomSourceConfig() : randomInvalidSourceConfig() : randomSourceConfig();
    }
    @Override
    protected SourceConfig mutateInstance(SourceConfig instance) {
        return null;
    }
    @Override
    protected boolean supportsUnknownFields() {
        return lenient;
    }
    @Override
    protected Predicate<String> getRandomFieldsExcludeFilter() {
        
        return field -> field.isEmpty() == false;
    }
    @Override
    protected Reader<SourceConfig> instanceReader() {
        return SourceConfig::new;
    }
    public void testGetRuntimeMappings_EmptyRuntimeMappings() {
        SourceConfig sourceConfig = new SourceConfig(
            generateRandomStringArray(10, 10, false, false),
            QueryConfigTests.randomQueryConfig(),