[Filebeat] Support mixed line terminators

[phamour]

Describe the enhancement:
By #5500 and PR #11015, filebeat started supporting line terminator config by choosing only one from candidate values like carriage_return, line_feed, etc. IMHO it would be much better to specify line terminator by regex or at least choosing multiple/mixed predefined options like \r or \r\n or \n.

Describe a specific use case for the enhancement or feature:
I'm using filebeat in k8s to gather logs from pods. In these pods there are logs generated by progressbar tools like tqdm that uses \r to separate progress messages. Instead of reading each of these progress messages as a separated line, filebeat's harvester gather all progresses as a whole, so that output is not sending in real time to downstream elasticsearch. If I configure the line terminator as \r, the harvester no longer works with normal logs ended by \n.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!