Add E2E to verify GPGP commits signing via UI

Commits made to a GitLab repository can be signed using GPG.

If GitLab can verify the commit signature, the commit will be marked as Verified when viewed in the UI. However, when using the UI, commits do not get signed by the user making the change - instead Gitaly can be configured to to sign commits made with the GitLab UI - these commits are marked as Unverified in the UI.

If Gitaly is not configured with a GPG signing key, commits will not be marked with either Verified or Unverified when commits are made via the UI.

We should add a number of E2E tests to verify this functionality. Scenarios that may be applicable may include:

• Commit via CLI https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/347785
• Commit via HTTP API (need to clarify if we can sign a commit pushed via HTTP)
• Commit via Web editor (signed and unsigned)
• Commit via Web IDE (signed and unsigned)
• Commit via merge requests (signed and unsigned)

https://docs.gitlab.com/ee/user/project/repository/gpg_signed_commits/
https://docs.gitlab.com/ee/administration/gitaly/configure_gitaly.html#configure-commit-signing-for-gitlab-ui-commits