Add authentication token to Error Tracking client

What does this MR do and why?

• Add authentication token to the error tracking gem and bundled client.
• Change error tracking internal endpoint to stop using decode64 for the token
• Fix generated DSN for opstrace based error tracking

Fixes gitlab-org/opstrace/opstrace#1749 (closed)

Depends on: gitlab-org/opstrace/opstrace!1688 (closed) and gitlab-org/opstrace/opstrace!1675 (merged)

How to set up and validate locally

Enable the feature flag gitlab_error_tracking and use_click_house_database_for_error_tracking:

Feature.enable(:gitlab_error_tracking)
Feature.enable(:use_click_house_database_for_error_tracking, project)
Feature.enable(:integrated_error_tracking)

1. Go to /admin page
2. Find GitLab Error Tracking in "General"
3. Enable the boolean and set the URL
4. Save settings

Retreive the token from that page and define it in Opscode gatekeeper.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.