Add support for plugins / predefined formats.

[tigrannajaryan]

Support Stanza-like plugins for log format, see https://github.com/observIQ/stanza/blob/master/docs/plugins.md

Preferably this should be a generic mechanism for importing templated configurations that can be used not just with logs but for other data types, and generally with any configuration section.

Details TBD.

[bogdandrutu]

I think this is a "config" change, so probably needs to be in core?

[github-actions]

This issue has been inactive for 60 days. It will be closed in 60 days if there is no activity. To ping code owners by adding a component label, see Adding Labels via Comments, or if you are unsure of which component this issue relates to, please ping @open-telemetry/collector-contrib-triagers. If this issue is still relevant, please ping the code owners or leave a comment explaining why it is still relevant. Otherwise, please close it.

[github-actions]

This issue has been closed as inactive because it has been stale for 120 days with no activity.