[IDM-232] - feat: stripe billing portal session

[kiremitrov123]

Description

This pull request implements the backend logic for creating a billing portal session within the Access Bridge component. It establishes the necessary interactions with the Sims and Stripe services to validate the viewer and create a checkout session.

Endpoint: POST: /v2/billing-portal

Jira card: Stripe Billing Portal URL Design document: AC-SIMS integration

According to our definition of done, I have completed the following steps:

• [x] Acceptance criteria met
• [x] Unit tests added
• [x] Docs updated (including config and env variables)
• [ ] Translations added
• [ ] UX tested
• [ ] Browsers / platforms tested
• [x] Rebased & ready to merge without conflicts
• [x] Reviewed own code

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit d5cff56):

https://ottwebapp--pr601-idm-232-stripe-billi-b7a25yt7.web.app

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