Support nested/grouped response format in query results

**Is your feature request related to a problem? Please describe.**

I'm always frustrated when querying Cube by a dimension (lets say govtScheme) and having to manually transform the flat tabular response into a grouped/nested object structure on the client side for every project.

**Describe the solution you'd like**
An optional query parameter (e.g. groupBy) that tells Cube to return results nested by a specified dimension:
```
{
"NIDESP": [{ "name": "Business Plan A" }],
"JANDHAN": [{ "name": "Business Plan B" }]
}
```

**Describe alternatives you've considered**

Manually reshaping the flat response on the client side using Array.reduce(). This works but adds repetitive boilerplate code across every application consuming the Cube API.

**Additional context**

This would be especially useful for dashboards and reporting tools where data is naturally presented in grouped sections. It reduces client-side complexity and makes Cube's REST API response more directly usable.

No file, test, or code entry point is named. Start by locating Cube's REST API query-result handling and the existing query-parameter flow; define the grouping behavior and verify that the response matches the nested example for the requested dimension.

Written by the indexing model from the issue text.