add support for side-by-side runtime selectors

[yaauie]

When included with:

    include LogStash::PluginMixins::ECSCompatibilitySupport(:disabled,:v1)

• Plugins will prevent instantiation in unsupported ecs_compatibility modes.

• Plugin instances will have an ecs_select method that allows in-line definition of alternates that resolve at runtime to the effective ecs_compatibility mode:

   def register
     @field_hostname = ecs_select[disabled: "hostname", v1: "[host][name]"]
     @field_hostip   = ecs_select[disabled: "ip",       v1: "[host][ip]"  ]
   end

  Usage of the ecs_select method safeguards against a developer failing to specify all alternates, which makes it easy to build large tables like those required in the CEF codec's 50+ field mappings.

• Plugin specs will be able to use a SpecHelper that provides ecs_compatibility_matrix helper for setting up a matrix of supported modes for testing. The method yields an Selector::State, which can be used just like ecs_select:

  require 'logstash/plugin_mixins/ecs_compatibility_support/spec_helper'
  
  describe YourClass, :ecs_compatibility_support
   ecs_compatibility_matrix(:disabled, :v1) do |ecs_select|
     subject(:instance) { described_class.new(ecs_compatibility) }
     its(:mode) { is_expected.to be ecs_compatibility }
     its(:description) { is_expected.to eq ecs_select[disabled: "legacy", v1: "novel"] }
   end
  end

  Which produces the following output in rspec's documentation mode:

  YourClass
   `ecs_compatibility => disabled`
     mode
       is expected to equal :disabled
     description
       is expected to eq "legacy"
   `ecs_compatibility => v1`
     mode
       is expected to equal :v1
     description
       is expected to eq "novel"