elasticsearch output: support external document versioning #4893

Problem

The elasticsearch_v8 / elasticsearch_v9 outputs can set a document _id and an action, but cannot set Elasticsearch's per-document external version. Each bulk op is built with only Index_, Id_, Pipeline, Routing, so the outputs always use ES internal versioning, hence last-writer-by-arrival. Internal versioning works well when writes to a given _id arrive in order, but it's less well suited to streaming ingest where the same _id is written repeatedly and correctness depends on which version wins, not which arrived last:

• Out-of-order delivery with max_in_flight > 1 (concurrent bulks, per-shard fan-out) can apply two writes to the same _id in the wrong order.
• At-least-once redelivery on replay an older offset document over a newer one.
• Concurrent writers race, and internal versioning can't arbitrate by source order.

Proposal

Add an opt-in, interpolated version field. When set, the output assigns an external version (version_type: external, strict greater-than) to whole-document write actions (index, upsert, create, delete). ES applies the write only if the supplied version beats the stored one; otherwise it returns 409, which the output treats as a successful no-op making ingest idempotent and order-independent at the datastore. Fully opt-in (unset ⇒ remains current behavior).
Not applicable to update (update API uses internal versioning + retry_on_conflict).

Example

output:
  elasticsearch_v8:
    urls: ['http://localhost:9200']
    index: "things"
    action: "index"
    id: ${! metadata("kafka_key") }
    # Use the source record's timestamp as the version, so a later revision of
    # the same key always wins. Any monotonic per-key value works — a value from
    # a record header does too, e.g.
    #   version: ${! metadata("event_version") }
    version: ${! metadata("kafka_timestamp_ms") }

With this config, replaying an older record for an existing id is a no-op (Elasticsearch returns 409, which the output treats as success) instead of overwriting the newer document.

Open items / possible follow-ups

• external_gte: This iteration scopes to version_type: external (strict >). external_gte (allow equal-version rewrites) could be added later if there's demand.

Implementation is ready; happy to open the PR :)