Integrate with now-u-backend (api-v2)

[JElgar]

Description

• Start migrations to stacked 3
• Migrate to api v2, with causes_service, supabase and meilisearch
• Generate api client using openapi generator

There are a lot of TODOs off the back of these changes but this is good enough for a beta release

Checklist:

Creator

• The target branch is dev
• I have updated the unreleased section of the change log
• I have reviewed the 'files changed' tab on github to ensure all changes are as expected
• I have added someone to review the pr

Reviewer

• I have verified the above are all completed
• I have run the code locally to ensure it fufills the requirements of the issue
• I have reviewed the 'files changed' and commented on any sections which I think are not needed, incorrect or could be improved

After pull

• If appropriate I have closed the issue/ moved the trello card