Switched capture strategy directly adding spy appender

[andsel]

Reworked log capture in setting_with_deprecated_alias_spec to use Logger's addAppender and a spy appender implementation.
This leverages the idea to don't reconfigure the Log4J with a custom build configuration, but simply adding an appender that could spy the appended lines to a logger, like described in https://github.com/andsel/log4j2_log_spy?tab=readme-ov-file#idea-2---add-a-customer-appender-to-the-logger-to-collect-the-messages.

The idea is use a list as a message accumulator, but the strange thing is that a message just added disappear and re-appear without an apparent motivation.

org.logstash.RSpecTests > rspecTests[core tests] STANDARD_OUT
    Run options: exclude {:integration=>true, :redis=>true, :socket=>true, :performance=>true, :couchdb=>true, :elasticsearch=>true, :elasticsearch_secure=>true, :export_cypher=>true, :windows=>true}

    Randomized with seed 64075

    LogStash::Setting::SettingWithDeprecatedAlias
      Settings#get on deprecated alias
    DNADBG>> before - before calling log.info, events: ["12345"]
    DNADBG>> CustomAppender.append invoked log_event: Logger=org.logstash.settings.DeprecatedAlias Level=INFO Message=Test ping message
    DNADBG>> CustomAppender.append invoked list content: ["12345", "Test ping message"]
    DNADBG>> before - just after called log.info, events: ["12345"]
    DNADBG>> inside the test
    DNADBG>> CustomAppender.append invoked log_event: Logger=org.logstash.settings.DeprecatedAlias Level=WARN Message=The value of setting `canonical.setting` has been queried by its deprecated alias `legacy.setting`. Code should be updated to query `canonical.setting` instead
    DNADBG>> CustomAppender.append invoked list content: ["12345", "Test ping message", "The value of setting `canonical.setting` has been queried by its deprecated alias `legacy.setting`. Code should be updated to query `canonical.setting` instead"]
    DNADBG>> inside the test - after exercise
        produces a WARN-level message to the logger (FAILED - 1)

    Failures:

      1) LogStash::Setting::SettingWithDeprecatedAlias Settings#get on deprecated alias produces a WARN-level message to the logger
         Failure/Error: expect(events[0].message.formatted_message).to include("setting `#{canonical_setting_name}` has been queried by its deprecated alias `#{deprecated_setting_name}`")

         NoMethodError:
           undefined method `message' for nil:NilClass
         # ./logstash-core/spec/logstash/settings/setting_with_deprecated_alias_spec.rb:199:in `block in <main>'
         # ./spec/spec_helper.rb:84:in `block in <main>'
         # ./logstash-core/lib/logstash/util.rb:43:in `set_thread_name'
         # ./spec/spec_helper.rb:83:in `block in <main>'
         # ./spec/spec_helper.rb:76:in `block in <main>'
         # ./vendor/bundle/jruby/3.1.0/gems/logstash-devutils-2.6.2-java/lib/logstash/devutils/rspec/spec_helper.rb:47:in `block in <main>'
         # ./lib/bootstrap/rspec.rb:36:in `<main>'

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    1 example, 1 failure