E2E: Add pipeline to run with gitaly feature flags
What does this MR do and why?
Adds an E2E test pipeline that runs Gitaly with all Feature Flags enabled.
This is achieved via use of an test environmental variable setting as documented in internal/featureflag/featureflag.go#L17-26
Corresponding change for gitlab-org/gitlab-qa: Enable gitaly feature flags by default (gitlab-qa!1227)
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Edited by John McDonnell