Resolve "Custom CI/CD configuration file doesn't allow manually triggering pipeline"

What does this MR do and why?

This change adds a "New pipeline" button to empty state screens that appear when a project uses external CI/CD configuration (like when the pipeline configuration is stored outside the GitLab repository).

Previously, when users viewed these empty states, they had no direct way to create a new pipeline. Now, if the user has permission to create pipelines, they'll see a button that takes them to the new pipeline creation page. The button only appears when appropriate - it's hidden if the user doesn't have the necessary permissions or if no pipeline creation path is available.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a new project and navigate to Settings -> CI/CD -> General pipelines
2. Select CI/CD configuration file and fill in the input with the existing .gitlab-ci.yaml file link
3. Navigate to Build -> Pipelines and verify the new pipeline button is present
4. Navigate to Build -> Pipeline editor and verify the new pipeline button is present

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #511296 (closed)