feat(otel): add skeleton of Beat processor

[andrzej-stencel]

Proposed commit message

Adds a no-op skeleton of a new OpenTelemetry Collector processor, Beat processor.

It is intended as a way to configure global Beat processors when running Beats as OTel receivers.

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
  • not much to test as the processor is currently no-op
• [ ] I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

See elastic/elastic-agent#8965 for including the processor in Elastic Agent.

Use cases

See README of the new processor.

[github-actions]

🤖 GitHub comments

Expand to view the GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

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

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @andrzej-stencel? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[VihasMakwana]

@andrzej-stencel Correct me if I'm wrong, but this would replace the global processors for beat receivers, right?

[rdner]

Looks good to me as a "skeleton".

[AndersonQ]

Looks good, just a NIT comment on the wording in the readme. It's optional, if don't wanna do it, it's fine.

[leehinman]

LGTM