Wire up the Apps Nav Tool Backend

The Apps Nav Tool created in #14815 is a React client-side app. Once it's hosted in the admin tool (#14840), we need it to be able to make requests to the backend to persist the data it creates. The data is currently managed by `cross-platform-navigation`[^1] in S3 in the mobile account, so we'd need an endpoint in the frontend admin app that can write to this bucket.

[^1]: https://github.com/guardian/cross-platform-navigation