Get all available avatars from HeyGen with pagination.

Args: limit: Number of avatars to return (1-100, default: 20) offset: Number of avatars to skip (default: 0)

Returns: Paginated list of available avatars with their IDs, properties, and pagination metadata.

Response format: { "success": true, "data": { "avatars": [...], "pagination": { "total": 150, "limit": 20, "offset": 0, "count": 20, "has_more": true } } }