Feat: introduce a target check helper

[kares]

for re-use across plugins - unifying the message, usage:

class LogStash::Filters::Json < LogStash::Filters::Base

  include LogStash::PluginMixins::ECSCompatibilitySupport
  include LogStash::PluginMixins::ECSCompatibilitySupport::TargetCheck

  extend LogStash::PluginMixins::ValidatorSupport::FieldReferenceValidationAdapter
  
  config :target, :validate => :field_reference
  # ...
  
  def register
    check_target_set_in_ecs_mode
    # ...
  end
  
end

[yaauie]

In general, this certainly is on track.

Thinking about this purely from a usage perspective, it would be very nice if a developer could include a single module into their plugin and not have to do anything else (such as overriding Plugin#register or modifying their existing override). That way we could validate the behaviour extensively here in this support gem, and consumers would simply need to validate the presence of this support gem in their plugin ancestors without also re-validating its behaviour at a unit level.

I've included some additional thoughts and pseudo-code in the diff.