Support plan for AVG for scatter queries in Gen4

[GuptaManan100]

Overview of the Issue

The following query returns incorrect result -

SELECT AVG(book6s.price) FROM book6s

The plan created from Gen4 is as follows -

{
  "QueryType": "SELECT",
  "Original": "select AVG(user.intcol) from user",
  "Instructions": {
    "OperatorType": "Aggregate",
    "Variant": "Ordered",
    "Aggregates": "count(0)",
    "Inputs": [
      {
        "OperatorType": "Route",
        "Variant": "SelectScatter",
        "Keyspace": {
          "Name": "user",
          "Sharded": true
        },
        "FieldQuery": "select AVG(`user`.intcol) from `user` where 1 != 1",
        "Query": "select AVG(`user`.intcol) from `user`",
        "Table": "`user`"
      }
    ]
  }
}

The plan is incorrect since we are running an AVG in SelectScatter. For an AVG query, we have to get all the rows from the shards and take the average on vtgate.

Reproduction Steps

Steps to reproduce this issue, example:

1. Copy the query to onecase.txt
2. Run the test TestOneCase

Binary version

• main

[harshit-gangal]

It does not add support for it. But it blocks from producing wrong plans.

#8593

[harshit-gangal]

PR: #14419