Save dependency list export file with .csv extension

What does this MR do and why?

This merge request updates the downloaded filename from the Explore / Dependency list export from dependencies.json to dependencies.csv It adds a new action to set the namespace type, which is used to determine the filename when downloading dependencies. It also updates the downloadExport action to use the correct filename based on the namespace type. Additionally, it adds some constants and mutation types for the new namespace type action.

#438743 (closed)

MR acceptance checklist

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

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:explore_dependencies)

2. Visit http://127.0.0.1:3000/explore/dependencies
3. Click on the "Export" button.
4. Verify that the downloaded file is named "dependencies.csv"