Switch to new DWS runway fleet via feature flag

What does this MR do and why?

This MR is a part of the snapshot migration of Duo Workflow Service. It introduces new_duo_workflow_service feature flag to switch to the new Duo Workflow Service runway fleet that is deployed from AI Gateway project. With this feature flag, we will test if the phase 2 - repository migration is ready to proceed.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/535545+ and #540641 (closed)

Side notes:

• Current DWS runway fleet is exposed to 443 port via Runway/CloudRun (fyi. internally port forwarded to 8080 in the container, which is exposed at PORT in the GRPC server). Since Cloud Connector (e.g. https://cloud.gitlab.com) and these intermediate layers use HTTPS (443) for the protocol by default, using Gitlab.config.cloud_connector.base_url to construct the endpoint to the DWS in GitLab-Rails is OK.
• New DWS runway fleet is the same configuration.
• Tested with curl -v https://duo-workflow.runway.gitlab.net and curl -v https://duo-workflow-svc.runway.gitlab.net, and confirmed that the requests reached via logs (example)
• This MR doesn't affect GDK installation. It affects only production and staging.
• Related to Allow configuring Duo Workflow Service URL in c... (!163565 - merged)

Screenshots or screen recordings

Before	After

How to set up and validate locally

When new_duo_workflow_service feature flag is disabled:

[6] pry(main)> Gitlab.config.duo_workflow.service_url
=> nil
[7] pry(main)> Gitlab.config.duo_workflow.secure
=> true
[8] pry(main)> Gitlab::DuoWorkflow::Client.url
=> "duo-workflow.runway.gitlab.net:443"
[9] pry(main)> Gitlab::DuoWorkflow::Client.secure?
=> true

When new_duo_workflow_service feature flag is enabled:

[7] pry(main)> Gitlab.config.duo_workflow.service_url
=> nil
[8] pry(main)> Gitlab.config.duo_workflow.secure
=> true
[9] pry(main)> Gitlab::DuoWorkflow::Client.url
=> "duo-workflow-svc.runway.gitlab.net:443"
[10] pry(main)> Gitlab::DuoWorkflow::Client.secure?
=> true

When service_url and secure is specified in gitlab.yml:

[1] pry(main)> Gitlab.config.duo_workflow.service_url
=> "gdk.test:50052"
[2] pry(main)> Gitlab.config.duo_workflow.secure
=> false
[3] pry(main)> Gitlab::DuoWorkflow::Client.url
=> "gdk.test:50052"
[4] pry(main)> Gitlab::DuoWorkflow::Client.secure?
=> false

MR acceptance checklist

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