Support custom registries for Foundational Flows

What does this MR do and why?

This MR adds support for setting custom registries for Foundational Flows in the instance admin menu of self-managed instances.

References

Screenshots or screen recordings

	Before	After
Self managed admin settings http://gdk.test:3000/admin/gitlab_duo/configuration
SaaS namespace settings http://gdk.test:3000/groups/gitlab-duo/-/settings/gitlab_duo/configuration

How to set up and validate locally

1. If not yet merged, rebase on the backend !222281 (merged)
2. Run GDK in self-managed mode
3. Go to instance admin settings: http://gdk.test:3000/admin/gitlab_duo/configuration
4. Add a registry e.g. "non-existing-registry.gitlab.com" and click "Save changes"
5. Go to a Duo enabled project and open e.g. an issue and click on "Generate MR with Duo"

6. Open the session, click on details and then on the job id

7. Observe that the job failed as it could not pull from the registry

8. Optionally clear the registry in the settings and run another flow observe that it pulls from registry.gitlab.com and passes.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #584689 (closed)