DocumentDB sync service and ChangeStreams feature gate

[WentingWu666666]

Summary

Add a Python-based change stream sync service that replicates data from a DocumentDB instance (created by the operator) to Azure DocumentDB (with MongoDB compatibility), along with operator changes to support change streams via a new feature gate.

Changes

Sync Service (new: documentdb-playground/sync_service/)

• sync.py: Collection-level change stream watcher with idempotent upsert/delete sync, exponential backoff retry, and graceful shutdown (SIGINT/SIGTERM)
• state.py: Crash-safe resume token persistence using atomic file writes (write-to-temp-then-rename), per-collection token tracking, and sync statistics
• config.yaml: Template configuration for source/target URIs and collection watch list
• requirements.txt: Python dependencies (pymongo, pyyaml)
• README.md: Full documentation including Azure DocumentDB creation via az cosmosdb mongocluster create, configuration, usage, troubleshooting, and architecture diagram

Operator - ChangeStreams Feature Gate

• documentdb_types.go: Added FeatureGates field with ChangeStreams gate to the DocumentDB CRD spec
• feature_gates_test.go: Unit tests for feature gate validation
• cnpg_cluster.go: Set PostgreSQL wal_level=logical when ChangeStreams gate is enabled
• cnpg_cluster_test.go: Tests for WAL level configuration based on feature gate
• CRD manifests: Updated documentdb.io_dbs.yaml in both config/crd/bases/ and documentdb-helm-chart/crds/
• zz_generated.deepcopy.go: Auto-generated deep copy for new types