Add 'enrich' operator for setting up enrich indices

[craigtaverner]

The recent work to benchmark ES|QL has benefitted greatly from the addition of the esql operator in #1791. However, the benchmarks that make use of the enrich command need enrich indices set up, and this is currently achieved using a complex set of multiple raw operations:

• Create enrich source index and ingest data
• Delete previous enrich policy
• Create new enrich policy
• Execute the enrich policy
• Use the enrich policy within an ingestion pipeline

The three steps for delete/create/execute of the enrich policy can be combined into a single enrich policy operation. This operation should combine the following raw steps:

Delete

    {
      "name": "delete-enrich-nyc_vendors",
      "operation-type": "raw-request",
      "ignore": [404],
      "include-in-reporting": false,
      "method": "DELETE",
      "path": "/_enrich/policy/nyc_vendors"
    }

Create

    {
      "name": "create-enrich-nyc_vendors",
      "include-in-reporting": false,
      "operation-type": "raw-request",
      "method": "PUT",
      "path": "/_enrich/policy/nyc_vendors",
      "body": {
        "match": {
          "indices": "nyc_vendors",
          "match_field": "id",
          "enrich_fields": [ "name" ]
        }
      }
    }

Execute

    {
      "name": "execute-enrich-nyc_vendors",
      "operation-type": "raw-request",
      "method": "POST",
      "path": "/_enrich/policy/nyc_vendors/_execute"
    }

Combined

The above operations could be combined into one:

    {
      "name": "setup-enrich-nyc_vendors",
      "operation-type": "enrich_policy",
      "policy_name": "nyc_vendors",
      "delete": true,                              // default to true
      "enrich_type": "match",             // could be default to 'match'
      "indices": "nyc_vendors",          // could be default to value of 'policy_name'
      "match_field": "id",
      "enrich_fields": [ "name" ]
    }

Multiple policies

An interesting alternative might be to allow the definition of multiple policies with a more flexible syntax:

    {
      "name": "setup-enrich-policies",
      "operation-type": "enrich_policy",
      "delete": true,
      "policies": {
        "nyc_vendors": {
            "match": {
              "indices": "nyc_vendors",
              "match_field": "id",
              "enrich_fields": [ "name" ]
          }
        },
        "nyc_payment_types_fares": {
          "match": {
            "indices": "nyc_payment_types",
            "match_field": "type",
            "enrich_fields": [ "name", "fare" ]
          }
        }
      }
    }

One advantage of this syntax is the body of the enrich policy matches exactly the body of the create part of the enrich policy REST request, so we could just pass through that part of the json object.