Gitaly: Allow enabling all feature flags

What does this Merge Request do and why?

For Gitaly you can set the GITALY_TESTING_ENABLE_ALL_FEATURE_FLAGS environment variable. This allows the user to override all feature flags.

This change embeds that setting and allows users to set it with gitaly.enable_all_feature_flags in gdk.yml.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.