Implement aggregate grouping for GraphQL Interface as outlined by NDC Spec

### component
c/v3-ndc-postgres

### Is your proposal related to a problem?
The current implementation of the Hasura NDC Postgres connector lacks support for aggregate groups, which are defined in the NDC connector specifications. This is a major obstacle when implementing data platforms and interactive user interfaces for statistical datasets, as these rely heavily on aggregation and grouping operations to produce meaningful insights.

### Provide a clear and concise description of what you want to happen.
Extend the connector’s query translation layer to generate SQL GROUP BY statements based on [NDC Spec](https://hasura.github.io/ndc-spec/specification/queries/grouping.html) aggregate group definitions.

### Describe alternatives you've considered
unclear

Start with the c/v3-ndc-postgres query translation layer and the NDC grouping specification linked in the issue. Compare the connector’s current aggregate handling with the specification and verify that aggregate-group definitions produce SQL GROUP BY statements; the issue names no files or tests, so completion should be validated against the specified behavior.

Written by the indexing model from the issue text.