[BUG] PPLTypeChecker not work on aggregation functions

[LantaoJin]

What is the bug?
PPLTypeChecker not work on aggregation functions:

How can one reproduce the bug?

PUT test001
{
    "mappings" : {
        "properties" : {
            "key" : {
                "type" : "keyword"
            },
            "text" : {
                "type" : "text"
            }
        }
    }
}
PUT test001/_doc/1
{
  "key": "abc",
  "text": "def"
}

POST /_plugins/_ppl/
{
  "query": """
  source=test001 | stats sum(text) 
  """
}
POST /_plugins/_ppl/
{
  "query": """
  source=test001 | stats sum(key) 
  """
}

Queries failed in codegen.

What is the expected behavior?
Throw error message that the sum() accepts numeric types only.

[LantaoJin]

@yuancu can you take a look the issue?

[yuancu]

@LantaoJin Copy