E2E test: tag e2e tests which use an external ai provider

What does this MR do and why?

• Adds external_ai_provider tag to tests that require a real ai provider

This is for two reasons:

1. It's useful to explicitly state what is required for these tests to work
2. We can use this as a tag if we only want to run ai-related tests (see: https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/5920#note_1852163767 / gitlab-org/quality/quality-engineering/team-tasks#2578)

How to set up and validate locally

cd qa
bundle exec rspec qa/specs/features/ee/ --tag external_ai_provider --dry-run

Should produce:

Create
  Code Suggestions
    when streaming code suggestions
      on SaaS
        behaves like completions API with PAT auth using streaming
          streams a suggestion
    when code completion is requested
      on SaaS
        behaves like completions API with PAT auth
          returns a suggestion
    when code generation is requested
      on SaaS
        behaves like completions API with PAT auth
          returns a suggestion

Ai-powered
  Duo Chat
    when initiating Duo Chat
      returns a response to a simple request

Create
  Duo Chat in Web IDE
    when initiating Duo Chat
      returns a response to a simple request

Create
  Code Suggestions in Web IDE
    on GitLab.com
      behaves like a code generation suggestion
        returns a code generation suggestion which can be accepted
      behaves like a code completion suggestion
        returns a code completion suggestion which can be accepted