Adds undercover gem to the stack

[filipefurtad0]

What? Why?

• Closes [CI] Signal test coverage changes (SimpleCov + Undercover gems) #13102

To make this effort work, we need to:

• add the undecover gem and make sure all dependencies are correctly installed
• add a task to the current build file to enable test coverage between a pull-requests' branch and the current master branch
• I think ideally, we would have a separate job, which passes/fails like all other jobs (preferred, as it would indicate if the branch worsens/improves test coverage)

What should we test?

• after pulling the branch and running bundle install, run bundle exec undercover --compare origin/master

  • it should output something like:
    " 🚨 WARNING: Coverage data is older than your latest changes and results might be incomplete. Re-run tests to update
    undercover: ✅ No coverage is missing in latest changes
    Undercover finished in 1.3279s"

• look at the CI run for this branch; find a new job, dedicated to undecover. It should display a comparison with master branch and pass/fail, accordingly.

Release notes

Changelog Category (reviewers may add a label for the release notes):

• User facing changes
• API changes (V0, V1, DFC or Webhook)
• Technical changes only
• Feature toggled

The title of the pull request will be included in the release notes.

Dependencies

Documentation updates