[8.14](backport #39873) Add Offset to libbeat/reader.Message

[mergify]

Proposed commit message

libbeat: add Offset to libbeat/reader.Message

This commit introduces the Offset property to libbeat/reader.Message, which stores the total number of bytes read and discarded before generating the message. The Offset field allows inputs to accurately determine how much data has been read up to the message, calculated as Message.Bytes + Message.Offset.

With this new Offset field, the filestream input correctly updates its state to account for data read but discarded by the include_message parser.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
  -~~ [ ] I have made corresponding change to the default configuration files~~
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

None

How to test this PR locally

Follow the instructions on #39653

Related issues

• Closes Filestream include_message do not correctly track the offset of a file #39653

---

This is an automatic backport of pull request #39873 done by [Mergify](https://mergify.com).

[mergify]

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Changes to be committed:
	modified:   CHANGELOG-developer.next.asciidoc
	modified:   CHANGELOG.next.asciidoc
	modified:   filebeat/input/filestream/input.go
	modified:   filebeat/input/filestream/parsers_integration_test.go
	modified:   libbeat/reader/filter/filter.go
	modified:   libbeat/reader/message.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   filebeat/input/filestream/environment_test.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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