Feature: Add file_multiline action to join Docker split container log chunks (~16KB)

Is your feature request related to a problem? Please describe.

When reading container logs via the file input plugin (Docker JSON log format, e.g. /var/log/containers/*.log), long log lines are split into multiple file records (~16KB per chunk). Each chunk becomes a separate event with a partial log field.

This breaks downstream processing: json_decode / decode fail on incomplete JSON, and fields like msg_body or logs_type are never parsed correctly.

The k8s input plugin already handles this via built-in k8s-multiline, but there is no equivalent for users who read the same files with the file input plugin.

Describe the solution you'd like

Add a new action plugin file_multiline that:

Joins sequential chunks from the same file stream into one event
Works on a configurable field (default: log)
Treats a chunk as complete when the field value ends with a real newline character \n
Supports split_event_size for very long messages (similar to k8s-multiline)
Is intended to run before json_decode / decode in the actions pipeline
Example:

actions:

• type: file_multiline
  field: log
  split_event_size: 1000000
• type: json_decode
  field: log
  Describe alternatives you've considered

join action with regex — not suitable: split chunks have no predictable start/continue pattern; content is arbitrary JSON/text inside log.

k8s input instead of file — works, but requires k8s-specific setup and metadata; not ideal when only file tailing is needed.

Pipeline event_timeout only — does not solve joining; only limits wait time for incomplete buffers.

Fixing only in application — avoids splitting at source, but does not help when logs are already written by Docker/CRI.

Start by examining the existing k8s-multiline implementation and the actions pipeline to understand event buffering and action configuration. Define how file_multiline tracks sequential chunks, newline completion, configurable fields, and split_event_size; it is done when Docker-split records are joined before json_decode or decode without affecting separate file streams.