### 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
if (runtimeMappings.isEmpty() == false) {
            builder.field(SearchSourceBuilder.RUNTIME_MAPPINGS_FIELD.getPreferredName(), runtimeMappings);
        }
        builder.endObject();
        return builder;
    }
    @Override
    public boolean equals(Object other) {
        if (other == this) {
            return true;
        }
        if (other == null || other.getClass() != getClass()) {
            return false;
        }
        SourceConfig that = (SourceConfig) other;
        return Arrays.equals(index, that.index)
            && Objects.equals(queryConfig, that.queryConfig)
            && Objects.equals(runtimeMappings, that.runtimeMappings);
    }
    @Override
    public int hashCode() {
        
        int indexArrayHash = Arrays.hashCode(index);
        return Objects.hash(indexArrayHash, queryConfig, runtimeMappings);
    }
    public static SourceConfig fromXContent(final XContentParser parser, boolean lenient) throws IOException {
        return lenient ? LENIENT_PARSER.apply(parser, null) : STRICT_PARSER.apply(parser, null);
    }
}