feat: Add subgraph sampling as reusable multi-hop subgraph sampling foundation

[aotenjou]

What changes were proposed in this pull request?

This PR introduces a model-agnostic subgraph sampling foundation for graph learning algorithms such as GCN and GNN and an inner implementation of #502.

Changes

• Add fanout-based one-hop neighbor sampling

  • fanout > 0 selects at most the requested number of neighbors
  • fanout = -1 returns all neighbors matching the requested direction
  • Parallel edges belonging to the same neighbor are retained as a group
  • Sampling supports seeds, sampling versions, and returned-edge limits

• Add layered sampled-subgraph assembly

  • Assemble vertices and edges by sampling depth
  • At the terminal depth, return only vertices and their features without reading or returning adjacent edges
  • Enforce sampled-node and sampled-edge limits
  • Reject neighborhoods from inconsistent snapshot versions

• Add stable logical edge identities

  • Identify logical edges using sourceId + targetId + label + time
  • Deduplicate incoming and outgoing storage replicas of the same logical edge
  • Preserve parallel edges with different labels or timestamps

• Add an iterative multi-hop BSP protocol

  • Define REQUEST, RESPOND, COMMIT_AND_REQUEST, and COMPLETE phases
  • Add sampling clocks, request/response messages, response collectors, and per-round state
  • Support empty request/response barriers
  • Cover complete two-hop and three-hop message schedules

• Integrate sampling with the DSL algorithm runtime

  • Provide one-hop sampling contexts for static and dynamic graphs
  • Forward initIteration and finishIteration lifecycle callbacks
  • Read from a stable window snapshot during sampling
  • Track neighborhood change versions to determine when cached state must be refreshed
  • Register both source and target vertices when processing edge changes

Testing

The following local validation has passed:

• 453 tests across the four affected modules under JDK 8
• 47 focused sampling tests after splitting the changes into atomic commits

Coverage includes:

• Fanout, direction, seed, and sampling-version behavior
• Unlimited sampling with fanout = -1
• Parallel-edge grouping and logical-edge deduplication
• Empty edge lists at the terminal depth
• Snapshot consistency and capacity limits
• Complete two-hop and three-hop BSP message schedules
• Dynamic neighborhood change-version persistence
• Static and dynamic runtime lifecycle forwarding

How was this PR tested?

• Tests have Added for the changes
• Production environment verified