feat(aws_s3): add sqs.on_missing_object to dead-letter missing objects

[peczenyj]

Description

The SQS-driven aws_s3 input currently has a single, hard-coded behaviour when an SQS notification points at an S3 object that cannot be downloaded because it no longer exists (NoSuchKey / 404): it logs a warning, acks the notification (DeleteMessage) and discards it. The notification can therefore never reach an SQS dead-letter queue.

That is the correct default when delete_objects is enabled (a redelivered notification for an already-deleted object is expected and harmless), but it is wrong for pipelines that need a missing object to be treated as a delivery failure and dead-lettered for inspection/replay.

This PR adds an advanced enum field sqs.on_missing_object:

• drop (default, the historical behaviour): warn, ack, discard.
• nack: log an error and return the notification to the queue (the existing nack path) so an SQS redrive policy can dead-letter it after maxReceiveCount.

To avoid a foot-gun, combining on_missing_object: nack with delete_objects: true is rejected at config-parse time, since a redelivered notification for an object this input legitimately deleted would otherwise be dead-lettered.

The default value preserves existing behaviour exactly, so this change is fully backward compatible.

Changes

• New sqs.on_missing_object string enum field (drop | nack), Advanced(), default drop.
• Config-parse guard rejecting nack + delete_objects: true.
• Missing-object branch in readSQSEvents now routes NoSuchKey to the nack path when nack is selected (ERROR log) instead of always dropping (WARN log).
• Unit test covering the default, both values, and the rejected combination.
• Regenerated aws_s3.adoc and a CHANGELOG entry.

Fixes #4474