[filebeat] support specify fields to be indexed

[UnderTreeTech]

Describe the enhancement:

Can filebeat add a new feature like promtail, user can specify what fields to be indexed, those don't be specified are handled as plain text default.

Describe a specific use case for the enhancement or feature:

In my scenario, almost every single line log has three fields: trace_id, level, timestamp. I wanna just index these three fields and the left parts are just stored as plain text, I believe this will make the search faster.

[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!

[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!