### 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
.requiresRemoteCluster()
        );
        assertTrue(
            new SourceConfig(
                new String[] { "index1", "remote2:index2", "index3" },
                QueryConfigTests.randomQueryConfig(),
                randomRuntimeMappings()
            ).requiresRemoteCluster()
        );
        assertTrue(
            new SourceConfig(
                new String[] { "index1", "index2", "remote3:index3" },
                QueryConfigTests.randomQueryConfig(),
                randomRuntimeMappings()
            ).requiresRemoteCluster()
        );
        assertTrue(
            new SourceConfig(
                new String[] { "index1", "remote2:index2", "remote3:index3" },
                QueryConfigTests.randomQueryConfig(),
                randomRuntimeMappings()
            ).requiresRemoteCluster()
        );
        assertTrue(
            new SourceConfig(new String[] { "remote1:index1" }, QueryConfigTests.randomQueryConfig(), randomRuntimeMappings())
                .requiresRemoteCluster()
        );
    }
}