Redirect new SaaS purchases to CDot purchase flow

What does this MR do and why?

Building upon https://gitlab.com/gitlab-org/gitlab/-/issues/461743+s, this MR addresses enabling the CustomersDot purchase flow for new customers, who do not have a BillingAccount set up yet in CustomersDot.

MR acceptance checklist

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

Screenshots or screen recordings

New subscription flow:

Scenario	Screenshot
Buy Premium/Ultimate subscription (FF off)
Buy Premium/Ultimate subscription (FF on)
Post purchase - GitLab billing page

Storage/Compute Minutes flow:

Scenario	Source	FF off	FF on
Storage flow
Compute Minutes flow

How to set up and validate locally

• Since we will depend on interaction between CustomersDot and GitLab, we would need the following requisites enabled:

1. Ensure GDK and CustomersDot are both running on your system.
2. Ensure the GitLab GraphQL proxy setup is completed for communication between the two.
3. Create a new user in GitLab, or use an existing user for whom we haven't purchased any subscriptions.

• Now we can test out the flow by:

1. Enable the migrate_purchase_flows_for_new_customers feature flag.
2. Create a group within GitLab and navigate to Settings -> Billing.
3. Click on either Upgrade to Premium or Upgrade to Ultimate and the new navigation should kick in taking the user to the CustomersDot purchase flow with the namespace selected:
4. Now disable the feature flag and repeat step 3, you should be navigated to the existing GitLab purchase flow:
5. We can follow the above steps within the Usage Quotas section for Storage and Pipeline Minutes purchase flow as well to redirect to CustomersDot when the feature flag is switched on (Added screenshots above for the respective flow).

Related to #466362