Backend API support for pagination

[ahkcs]

Description

This PR adds offset-based pagination support for PPL queries executed through the Calcite engine. Users can now paginate through large result sets by specifying pageSize and offset parameters in the request body

• Simple API: Pass pageSize and offset in the JSON request body
• Respects user intent: User-specified commands like head X from Y are honored first

API Design

Request Format

  POST /_plugins/_ppl
  {
    "query": "source=logs | where status='error' | sort timestamp",
    "pageSize": 100,
    "offset": 0
  }

Parameters

Parameter	Type	Default	Description
pageSize	int	0	Number of rows per page. 0 = pagination disabled
offset	int	0	Number of rows to skip (0-based)

Example: Paginating Through Results

  Page 1 (rows 0-99):
  { "query": "source=accounts | fields name, age | sort age", "pageSize": 100, "offset": 0 }

  Page 2 (rows 100-199):
  { "query": "source=accounts | fields name, age | sort age", "pageSize": 100, "offset": 100 }

  Page 3 (rows 200-299):
  { "query": "source=accounts | fields name, age | sort age", "pageSize": 100, "offset": 200 }

Check List

• [ ] New functionality includes testing.
• [ ] New functionality has been documented.
• [ ] New functionality has javadoc added.
• [ ] New functionality has a user manual doc added.
• [ ] New PPL command checklist all confirmed.
• [ ] API changes companion pull request created.
• [ ] Commits are signed per the DCO using --signoff or -s.
• [ ] Public documentation issue/PR created.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• [ ] Create PR with unit tests
• [ ] Post copyable unit tests in a comment

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[ahkcs]

Can you add some tests in CalciteExplainIT?

Is this query

POST /_plugins/_ppl
  {
    "query": "source=logs | where status='error' | sort timestamp",
    "pageSize": 100,
    "offset": 1000
  }

equals to

POST /_plugins/_ppl
  {
    "query": "source=logs | where status='error' | sort timestamp | head 100 from 1000"
  }

?

Added test cases in CalciteExplainIT

[ahkcs]

Is this PR ready for review now?

Not yet, it's currently a POC PR, still need to finalize design