Make the queue's public API accept interface{} instead of publisher.Event

[faec]

What does this PR do?

Finishes the refactor started in #31356, modifying the public queue API to accept and return interface{} pointers rather than publisher.Event, which will allow the new elastic shipper to use the libbeat queue for its internal storage.

Closes #31307

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
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

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• Start Time: 2022-05-23T21:13:09.070+0000

• Duration: 75 min 11 sec

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Test	Results
Failed	0
Passed	22204
Skipped	1931
Total	24135

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[faec]

This is ready for review now (the tests pass, locally at least)

[fearful-symmetry]

So, my LGTM is limited to the memqueue, since that's really the only part I understand, but it looks fine.