Get volume cost breakdown

Retrieve volume costs with detailed breakdown by cost type.

**Query Parameters:**
- `volume_id` (optional): Filter by specific volume
- `group_by` (optional): Set to "date" to aggregate all volumes by date
- `cost_type` (optional): Filter by cost type ("storage", "egress", "requests", "other", "custom_storage", "chargeable", "all")
- `start_date` (required): Start date in YYYY-MM-DD format
- `end_date` (required): End date in YYYY-MM-DD format (max 3 months from start)
- `page` (optional): Page number for pagination (default: 1)
- `limit` (optional): Items per page (default: 50, max: 100)

**Cost Types:**
- `storage`: AWS S3 storage costs
- `egress`: AWS Data transfer out costs
- `requests`: All AWS request costs (Tier 1 + Tier 2)
- `requests_tier1`: AWS PUT, POST, COPY, LIST requests
- `requests_tier2`: AWS GET, SELECT, HEAD requests
- `other`: Other AWS S3 related costs
- `custom_storage`: Our custom storage pricing ($0.00005/gb/hr)
- `chargeable`: Cost actually charged to users (custom storage only)
- `all`: All cost types (default)

**Examples:**
- Get all costs: `GET /volumes/costs?start_date=2025-01-01&end_date=2025-01-31`
- Get storage costs only: `GET /volumes/costs?cost_type=storage&start_date=2025-01-01&end_date=2025-01-31`
- Get daily aggregated costs: `GET /volumes/costs?group_by=date&start_date=2025-01-01&end_date=2025-01-31`