chore(no-release): Add integration test support using a real instance

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Note - this MR is purposefully created from a fork instead of the main repository, to ensure that forked pipelines work properly still.

This MR adds support for writing integration tests that run against a locally running GitLab instance, to validate that APIs created function properly. It includes not just the CI setup, but a set of helpers to write integration tests that check if integration tests are enabled, and a helper function example.

A new set of User integration tests have been created as well, proving the concept.