enhancement(reduce transform): add max_bytes configuration option

[PGBI]

Summary

Add a new optional max_bytes configuration option to the reduce transform that triggers a flush when the accumulated byte size of a reduced event group would exceed the configured threshold.

This complements the existing max_events option and provides more granular control over memory usage when reducing events. This will also help with sinks that have a max bytes limit, such as Datadog or Kinesis.

Vector configuration

my_transform_id:
    type: reduce
    inputs:
      - my-source-or-transform-id
    expire_after_ms: 30000
    flush_period_ms: 1000
    max_bytes: 1048576  # 1MB - flush when accumulated size would exceed this
    group_by:
      - request_id

How did you test this PR?

By adding unit tests covering the new option.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

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