Include list of tag in external pipeline validation api

What does this MR do and why?

Adding tag_list into External Pipeline Validation API

Connected to #335904 (comment 1818051865)

How to set up and validate locally

1. export EXTERNAL_VALIDATION_SERVICE_URL=<VALIDATOR_URL> (I'm using Public RequestBin)

2. gdk restart

3. Enable flag with Feature.enable(:ci_send_tag_list_for_external_validation)

4. Adding tag to .gitlab-ci.yml

   deploy1:
     tags:
       - test
     stage: deploy
     script:
       - echo "Do your deploy here"
     environment: production

5. Click Validate pipeline in Validate tab on Pipeline Editor Page

6. You should get this example request in RequestBin (You should see tag_list)

   {"project":{"id":7,"path":"flightjs/Flight","created_at":"2024-03-10T09:18:29Z","shared_runners_enabled":true,"group_runners_enabled":true},"user":{"id":1,"username":"root","email":"admin@example.com","created_at":"2024-03-10T09:17:50Z","current_sign_in_ip":"127.0.0.1","last_sign_in_ip":"127.0.0.1","sign_in_count":2},"credit_card":{"similar_cards_count":0,"similar_holder_names_count":0},"pipeline":{"sha":"ae2e9a0694f7b92a6c631556d30dedaffc2b4794","ref":"master","type":"push"},"builds":[{"name":"deploy1","stage":"deploy","image":null,"services":null,"script":["echo \"Do your deploy here\""],"tag_list":["gfedgfd"]}],"total_builds_count":0,"namespace":{"plan":"default","trial":false}}