Support nested aggregation

[LantaoJin]

Description

Support nested aggregation. With this PR, follow SQL are able to exectue a nested aggregation query.

SELECT min(nested(pages.load_time, pages)) FROM logs WHERE response = 200;

And it equals the DSL

GET logs/_search
{
  "query": {
    "match": { "response": "200" }
  },
  "aggs": {
    "pages": {
      "nested": {
        "path": "pages"
      },
      "aggs": {
        "min_load_time": { "min": { "field": "pages.load_time" } }
      }
    }
  }
}

Issues Resolved

PR resolves #2813 and #2529, and partly resolves #2739 since nested function hasn't supported in PPL yet.

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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