feat(routing): add HTTP policy provider with revision-aware caching

[JasonOA888]

Implements #812 - Support loading routing policy from external HTTP endpoint

Summary

• Add PolicyProvider module with HTTP endpoint fetching and caching
• Support policy_id + revision for revision-aware cache invalidation
• Add extract_routing_metadata() to extract policy_id/revision from requests
• Resolution order: inline policy > external provider > default
• Configurable TTL and custom headers for policy endpoint

Configuration

routing:
  policy_provider:
    url: "https://my-service.internal/v1/routing-policy"
    headers:
      Authorization: "Bearer $POLICY_API_KEY"
    ttl_seconds: 300

Request Format

{
  "messages": [...],
  "policy_id": "customer-abc-123",
  "revision": 42
}

Caching Behavior

• Cache key: policy_id with stored revision
• If cached revision >= requested revision, use cache
• Cache entries also expire via TTL as safety net

Flow

1. Request comes with policy_id and revision
2. Check local cache for policy_id
3. If cached and revision matches, use cached policy
4. Otherwise, fetch from HTTP endpoint
5. Validate response (policy_id match, schema_version supported)
6. Cache result and pass to router

Fixes #812