Add ingest-attachment support for per document indexed_chars limit

[dadoonet]

Note: this is a backport of #28977 which does not require a review.
As I forgot to backport it, I just like to double check that CI will be happy.

I'll update the PR if CI is unhappy and then will ask for a review.

---

We today support a global indexed_chars processor parameter. But in some cases, users would like to set this limit depending on the document itself.
It used to be supported in mapper-attachments plugin by extracting the limit value from a meta field in the document sent to indexation process.

We add an option which reads this limit value from the document itself
by adding a setting named indexed_chars_field.

Which allows running:

PUT _ingest/pipeline/attachment
{
  "description" : "Extract attachment information. Used to parse pdf and office files",
  "processors" : [
    {
      "attachment" : {
        "field" : "data",
        "indexed_chars_field" : "size"
      }
    }
  ]
}

Then index either:

PUT index/doc/1?pipeline=attachment
{
  "data": "BASE64"
}

Which will use the default value (or the one defined by indexed_chars)

Or

PUT index/doc/2?pipeline=attachment
{
  "data": "BASE64",
  "size": 1000
}

Backport of #28977 in 6.x branch (6.4.0)

[elasticmachine]

Pinging @elastic/es-core-infra

[martijnvg]

lgtm

[martijnvg]

@dadoonet The CI build failed because if a checkstyle violation: [ERROR] /var/lib/jenkins/workspace/elastic+elasticsearch+pull-request/plugins/ingest-attachment/src/main/java/org/elasticsearch/ingest/attachment/AttachmentProcessor.java:32:8: Unused import - java.io.IOException. [UnusedImports]