Implement a Kubernetes operator

We see strong customer demand for a Kubernetes Flagsmith operator.

People want to be able to:
- Manage post-deploy setup, including Flagsmith-on-Flagsmith configuration — see #4447.
- Create, update and configure projects (including SaaS) and associate them with deployments (e.g. using labels).
- Setup users and permissions.
- Create, update and configure environments.
- Create/update features and feature states (including SaaS) and associate them with deployments.

Our [Golang management API client](https://github.com/Flagsmith/flagsmith-go-api-client) has been recently updated to support the latter two. Research is necessary to determine how hard implementing the FoF setup would be.