ESQL: Allow access to _ignored meta field

[flash1293]

Description

The _ignored meta field (https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-ignored-field.html) is useful to gauge dataset quality by showing which field are not fully indexed.

However, it's currently not accessible via ESQL. It should be possible to include it via the METADATA clause (https://www.elastic.co/guide/en/elasticsearch/reference/current/esql-metadata-fields.html).

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[flash1293]

A related thing that should be exposed in some way as well is access to the ignored_field_values that are currently returned as part of _search:

 {
        "_index": "myindex",
        "_id": "S-FbTY8BbGXeaJt28wv3",
        "_score": 1,
        "_ignored": [
          "a"
        ],
        "_source": {
          "a": "abcsfdhjgfhjksdgfkjhgsdhfj"
        },
        "ignored_field_values": {
          "a": [
            "abcsfdhjgfhjksdgfkjhgsdhfj"
          ]
        }
      }

[bpintea]

Related: #108558