Add benchmark for processors that create event backups

[rdner]

This will come handy once we have another PR with optimizations.

Also, extended a test case in the append processor due to the lack of coverage.

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.

How to test this PR locally

cd ./libbeat/processors
go test -run=none -bench=".*EventBackups.*" -benchmem -benchtime=10x -memprofile mem-before.bin

On my machine I get:

goos: darwin
goarch: arm64
pkg: github.com/elastic/beats/v7/libbeat/processors
BenchmarkEventBackups/run_processors_that_use_backups-10                      10         336269333 ns/op        251485534 B/op    435076 allocs/op
PASS
ok      github.com/elastic/beats/v7/libbeat/processors  4.265s

336269333 ns/op 251485534 B/op 435076 allocs/op

As you can see, the current issue is that we clone the event every time and allocate a lot for this. This issue should be addressed by #36958

Related issues

• Relates Optimize memory allocations in the event processing pipeline #36958

[elasticmachine]

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