Ingest: Allow arrays in lowercase and uppercase processors

[fkelbert]

Elastic Cloud 7.5

Currently, we implement long lists of lowercase/uppercase processors to lowercase/uppercase a list of fields. These processors should be able to process multiple fields.

Potentially, this also applies to other processors.

Current approach:

{
  "processors": [
    {
      "lowercase": {
        "field": "mail.sender"
      }
    },
    {
      "lowercase": {
        "field": "mail.sender_domain"
      }
    }
  ]
}

Desired approach:

PUT _ingest/pipeline/generic
{
  "processors": [
    {
      "lowercase": {
        "field": [ "mail.sender", "mail.sender_domain" ]
      }
    }
  ]
}

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Ingest)