Resolve Alias Issues in Legacy SQL with Filters

[aparajita31pandey]

Description

Alias query in legacy SQL which had filters to it didn't work . This diff addresses that issue. This diff derives index name from alias, for legacy sql queries having filters to it. I plan to add testcases to this PR too

Before the change

curl -X POST "localhost:9200/_plugins/_sql" -H 'Content-Type: application/json' -d '{
  "query": "SELECT * FROM my-alias",
  "fetch_size": 1,
   "filter" : {
    "term" : {
      "field2" : "Sample keyword2"
    }
  }
}'
{
  "error": {
    "reason": "There was internal problem at backend",
    "details": "Index type [my-alias] does not exist",
    "type": "IllegalArgumentException"
  },
  "status": 500
}

After the change:

curl -X POST "localhost:9200/_plugins/_sql" -H 'Content-Type: application/json' -d '{
  "query": "SELECT * FROM my-alias",
  "fetch_size": 1,
   "filter" : {
    "term" : {
      "another_field" : "hello"
    }
  }
}'
{
  "schema": [
    {
      "name": "new_field",
      "type": "text"
    },
    {
      "name": "another_field",
      "type": "keyword"
    }
  ],
  "total": 1,
  "datarows": [[
    "Some text",
    "hello"
  ]],
  "size": 1,
  "status": 200

Related Issues

Resolves #2912
#1398

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[ctellechea2001]

@aparajita31pandey Do you know when could be this PR reviewed and merged to fix this issue that we have reported?
#2912