Redirect existing customers in Compute Minutes / Storage purchase flow

What does this MR do and why?

This MR builds upon the changes introduced in !157042 (merged), to add redirection to the following flows as well:

• Storage purchase flow
• CI Minutes purchase flow

Note: This MR is dependant on the changes from !157042 (merged) and should be merged after its predecessor.

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

Type	Pre-purchase	Redirection	Post-purchase
Storage flow
Compute Minutes flow

Storage purchase flow (FF off)	Minutes purchase flow (FF off)

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. Purchase any new subscription and ensure it is activated on a GitLab group.

• Now we can test out the flow by:

1. Enable the migrate_purchase_flows_for_existing_customers feature flag.
2. Create a group within GitLab and navigate to Settings -> Usage Quotas.
3. On the storage tab, click on Buy Storage and the new navigation should kick in taking the user to the CustomersDot purchase flow with the namespace selected.
4. Similarly, the CDot navigation will kick in on clicking on Buy additional compute minutes within the Pipelines tab.
5. Now switch off the feature flag from step 1, and the existing purchase flow within GitLab should show up.

Related to #461742