[Meta] libbeat refactorings and cleanup

[simitt]

As discussed with @urso this meta issue should be used to collect improvements for libbeat that exceed the scope of cleanup within other PRs. The meta issue should help identifying where code improvements are necessary and not forget about them.

• Get rid of using NewBeat returning a half initialized beat, in favor of NewInitializedBeat that ensures full initialization before returning the beat. NewInitializedBeat loads the beat config file and panics if this config is not available. Tests need to be updated to work with the initialization as unit tests by default don't provide the config file. Moreover NewBeat is an exported method, so it should only be deprecated until the next major version upgrade. Reference: Refactor export and idxmgmt handling. #11777 (comment)
• Evaluate and use Elastic's Golang Elasticsearch client instead of ours: https://github.com/elastic/go-elasticsearch

[simitt]

Closing this issue, as it didn't get updated with new issues and doesn't seem useful.