chore: Migrate external pipeline validation service

What does this MR do and why?

Add external_pipeline_validation_migration feature flag to gradually cut-over traffic for external pipeline validation service deployed in Runway. For gitlab-com/gl-infra/platform/runway/team#135 (closed).

MR acceptance checklist

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Test coverage has been added. Additionally, you can locally validate private method:

1. In rails console enable the experiment fully

   Feature.enable(:external_pipeline_validation_migration)

2. In rails console set environment variable

   ENV['EXTERNAL_VALIDATION_SERVICE_RUNWAY_URL'] = 'https://runway.validation-service.external/'

3. In rails console confirm validation service URL

   Gitlab::Ci::Pipeline::Chain::Validate::External.new(Ci::Pipeline.new, Gitlab::Ci::Pipeline::Chain::Command.new).send(:validation_service_url)
   # => "https://runway.validation-service.external/"