feat: Add optional /children route (Children STAC API Extension)

[jonhealy1]

Description

This feature request proposes adding support for the STAC API - Children Extension to stac-fastapi-elasticsearch-opensearch.

This extension adds a /children endpoint that provides a list of all child Catalogs and Collections for a given resource. While our primary architectural focus for federation is the Catalogs Endpoint Extension (/catalogs), supporting /children provides compliance with standard STAC crawlers (like STAC Browser) and offers a standardized mechanism for simple link-based traversal.

Motivation

• Interoperability: Enables standard STAC clients (e.g., STAC Browser) that rely on link crawling to navigate the hierarchy effectively.
• Navigation Parity: Provides a complementary "Browse" view alongside the "Search/Manage" view provided by the /catalogs endpoint.
• Community Alignment: Implements a widely recognized extension for hierarchical discovery.

Proposed Implementation

The implementation should be optional, controlled by an environment variable (e.g., ENABLE_CHILDREN_ROUTE, default false).

1. Root Endpoint (GET /children)

• Returns a list of all top-level Catalogs and Collections.
• Logic: Query the collections index for resources where direct_parents (or equivalent field) indicates the root.

2. Recursive Endpoints (Deep Traversal)
To support deep browsing, the implementation should support the "Relative Traversal" pattern:

• GET /catalogs/{catalog_id}/children
• GET /collections/{collection_id}/children (for nested collections)

3. Type Filtering (Recommended)
To ensure backend efficiency and predictable responses, the implementation should support the type parameter (as proposed in recent discussions):

• GET /children?type=Catalog
• GET /children?type=Collection

Database Considerations

Since SFEOS uses a flattened index strategy, implementing /children requires efficient querying of immediate descendants only.

• We cannot use the recursive parent_ids field (which contains the full lineage).
• We likely need to utilize a specific field (e.g., direct_parents) to filter for immediate children without returning the entire recursive sub-tree.

Acceptance Criteria

• The /children endpoint is available when enabled via config.
• The root landing page includes a rel="children" link.
• The endpoint returns a valid Children response object containing mixed or filtered Catalog and Collection objects.
• Supports the type query parameter for filtering.
• (Optional) Supports pagination consistent with other list endpoints.