Add tag list for external validation

What does this MR do and why?

This MR is add tags to External Pipeline Validation API, issue #335904 (closed)

Screenshots or screen recordings

n/a

How to set up and validate locally

gdk start

bundle exec rspec spec/lib/gitlab/ci/pipeline/chain/validate/external_spec.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.