Add semantic of Sample/Limit per vertex exploration/GetNeighbors

[wey-gu]

Is your feature request related to a problem? Please describe.
In GNN or other application use cases like (get 100 players and explore to get their followers, limit 2 per each )

Describe the solution you'd like

This is feasible/doable as the GetNeighbors limit pushdown is ready, right?

Could be SAMPLE/LIMIT before YIELD? like:

• This is to SAMPLE over single GetNeighbors

lookup on player YIELD id(vertex) AS VertexID | limit 5 | GO FROM $-.VertexID OVER follow SAMPLE [2] YIELD $^.player.name AS album, properties($$) AS photo 

• This is to SAMPLE overall on this step:

lookup on player YIELD id(vertex) AS VertexID | limit 5 | GO FROM $-.VertexID OVER follow  YIELD $^.player.name AS album, properties($$) AS photo SAMPLE [10]

Describe alternatives you've considered

Or use different keywords in the above position?

Additional context

• gnn: https://discuss.nebula-graph.com.cn/t/topic/406
• Support of cypher subquery #4081 cover some of the requirements from this case