Add GitLab DSN to error tracking UI

What does this MR do and why?

Given error tracking is enabled, and GitLab backend is choosen. User needs a DSN (string) that they can copy-paste into Sentry SDK so integration is working. Before this merge request users can only see DSN from API https://docs.gitlab.com/ee/operations/error_tracking.html#how-to-enable.

This merge request brings DSN to UI in a most simple way.

Issue #329596 (closed)

Screenshots or screen recordings

Before

After

How to set up and validate locally

Just open any project in GDK, select error tracking settings as shown on screenshot and press "Save changes"

MR acceptance checklist

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