Extract customers dot client creation to remove duplication

What does this MR do and why?

Extracts client creation for Customers Portal to remove duplication

Original discussion: !109277 (comment 1250780168)

Screenshots or screen recordings

Scenario	Screenshot
Addon purchase
Subscription table on Billing page
Invite members modal

How to set up and validate locally

1. Run GDK in SaaS mode
2. Create a new group

Addon purchase

1. Go to Usage Quotas page for this group
2. Go to Pipelines tab
3. Click on Buy additional minutes CTA

Subscription table on Billing page

1. Go to Billing page for this group

Invite members modal

1. Add a paid subscription for one seat
2. Visit this group page, then Group Information -> Members http://localhost:3000/groups/<your-group>/-/group_members.
3. Click on Invite members CTA.
4. Add a member in a dropdown and click Invite.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #388075