Add admin settings section

What does this MR do and why?

Related to Add admin settings section (#375577 - closed)

Adds a new setting section for GitLab Ultimate to set up Product Analytics configurations related to Jitsu and Cube.

Screenshots or screen recordings

Settings section

How to set up and validate locally

1. Make sure you are on GitLab Ultimate.
2. Enable the cube_api_proxy feature flag

   Feature.enable(:cube_api_proxy)

3. Go to your instance General admin settings at http://127.0.0.1:3000/admin/application_settings/general
4. Set valid values and click "Save changes".

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.