Add social network benchmark for async execution

This benchmark provides a realistic test of async GraphQL execution with:
- Nested object types (User -> Posts -> Comments -> Author)
- List fields with multiple items
- All async resolvers to properly test the async execution path
- Various query complexities from simple (4 fields) to complex (150+ resolvers)

Test cases:
- simple: Single user with basic scalar fields
- medium: User with posts (5 posts, 5 fields each)
- network: User with followers and following lists
- deep: 4-level nested query
- complex: User -> posts -> comments -> authors
- feed: 10 posts with authors and comments (typical feed query)

Author: claude