[WebIDE] Use GitLab instance as the Web IDE's extension host domain

Problem to solve

GitLab Self-Managed instances that run on air-gapped environments can't connect to the .gitlab-static.net extension host domain therefore many capabilities in the Web IDE don't work. .gitlab-static.net is a separate web service from GitLab instances which makes the task of providing an alternative solution very challenging for Gitlab Self-Managed instances administrators.

Solution proposal

Allow GitLab instance administrators to use the GitLab instance's static assets HTTP server as a replacement for .gitlab-static.net when instance administrators provide a custom extension host domain in application settings.

Acceptance criteria

• Replace hardcoded Web IDE CSP rules that point to gitlab-static.net with the custom extension host domain application setting.
• Replace hardcoded Web IDE CORS rules that point to gitlab-static.net with the custom extension domain application setting.
• Ensure that Workhorse delegates CORS headers resolution to the Rails Web server.
• Verify that this solution on work in an environment that uses mutual TLS.

Implementation ideas