Support COUNT star and literal in new engine

[dai-chen]

Issue #, if available: N/A

Description of changes: Support COUNT(*) and COUNT(1) in the new SQL engine.

1. Convert * to unresolved AllFields and then resolved literal "*". This is because COUNT(*) has same semantic as COUNT(any literal). So there is no need to add a new expression class (SELECT * has its own logic and don't need resolved AllFields expression neither)
2. Follow our old approach in legacy code by generating Value Count aggregation DSL on metadata field _index for COUNT(*) and COUNT(1).

Example:

POST _opendistro/_sql/_explain
{
  "query": """
    SELECT
      COUNT(*), COUNT(FlightNum), COUNT(1)
    FROM kibana_sample_data_flights
  """
}
{
  "root": {
    "name": "ProjectOperator",
    "description": {
      "fields": "[COUNT(*), COUNT(FlightNum), COUNT(1)]"
    },
    "children": [
      {
        "name": "ElasticsearchIndexScan",
        "description": {
          "request": """ElasticsearchQueryRequest(indexName=kibana_sample_data_flights, sourceBuilder={"from":0,"size":0,"timeout":"1m","aggregations":{"COUNT(1)":{"value_count":{"field":"_index"}},"COUNT(*)":{"value_count":{"field":"_index"}},"COUNT(FlightNum)":{"value_count":{"field":"FlightNum"}}}}, searchDone=false)"""
        },
        "children": []
      }
    ]
  }
}

Documentation: https://github.com/dai-chen/sql/blob/support-count-star-and-literal/docs/user/dql/aggregations.rst#count-aggregations

Testing:

1. Add new UT and comparison IT.
2. Ignore one failed IT PrettyFormatResponseIT.aggregationFunctionInSelectCaseCheck(). Please see the reason on @Ignore message.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[penghuo]

Thanks for the change!