Add an option to pre-create `FLAGSMITH_ON_FLAGSMITH` environment

One of the few things that are not yet automated when setting up self-hosted Flagsmith is creating and hooking up to the FoF environment.

It does make sense to perform this task as close to app code as possible, so we might make this part of Flagsmith's `bootstrap` management command, but it might be tricky since it involves modifying the app environment. Other option would be to build a Kubernetes operator to encapsulate bootstrapping tasks that require changes to the environment.

## Related issues

- #2077