Support nested collection/path queries for Firestore and RTDB

[omer-ayhan]

Feature Description

Add support for querying nested paths:

• Firestore: subcollections (e.g., users/{uid}/orders) and optionally collection group queries (if supported by the CLI design)
• RTDB: deep child paths (e.g., /root/a/b/c) with filters applied at the correct node

Proposed CLI / UX

• Allow specifying a nested path/collection target clearly (examples to be defined).
• Document limitations (Firestore index requirements, collectionGroup constraints, etc.).

Acceptance Criteria

• Firestore:
  • Can query a subcollection path correctly (filters + pagination where applicable).
  • If collectionGroup is supported, it works and is documented (including index requirements).
• RTDB:
  • Can query at a nested path and apply orderBy* + equalTo/startAt/endAt correctly.
• Includes tests (integration preferred) for both Firestore and RTDB nested queries.
• Docs updated with examples.

Additional Context

Implementation Subtasks

1. Requirements & CLI syntax design
2. Firestore query implementation (subcollection + optional collectionGroup)
3. RTDB nested-path query support
4. Tests
5. Docs