Backport PR #16786 to 8.17: Avoid lock when ecs_compatibility is explicitly specified

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Backport PR #16786 to 8.17 branch, original message:

---

Release notes

• Improves concurrent performance of some plugins when they are instantiated with an explicit ecs_compatibility directive.

What does this PR do?

Properly memoizes an explicitly-given ecs_compatibility directive on first access so that it can be retrieved without lock contention.

Because a break escapes a begin...end block, we must not use a break in order to ensure that the explicitly set value gets memoized to avoid lock contention.

def fake_sync(&block)
  puts "FAKE_SYNC:enter"
  val = yield
  puts "FAKE_SYNC:return(#{val})"
  return val
ensure
  puts "FAKE_SYNC:ensure"
end

returned = fake_sync do
  @ivar = begin
    puts("BE:begin")
    break :break # expectation: break the current `begin`...`end` block
  	
    val = :returned
    puts("BE:return(#{val})")
    val
  ensure
    puts("BE:ensure")
  end
end
puts({:returned => returned, :@ivar => @ivar})

Note: no FAKE_SYNC:return:

╭─{ rye@perhaps:~/src/elastic/logstash (main ✔) }
╰─● ruby break-esc.rb
FAKE_SYNC:enter
BE:begin
BE:ensure
FAKE_SYNC:ensure
{:returned=>:break, :@ivar=>nil}
[success]

Why is it important/What is the impact to the user?

Eliminates unnecessary lock contention, which can prevent some plugins from running as concurrently as they would otherwise be able.

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 (and/or docker env variables)
• [ ] I have added tests that prove my fix is effective or that my feature works

How to test this PR locally

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