Integrate core auth in EE

[foot]

• Core serves the UI from the api server
• EE serves the UI from another server that is mux'd by an ingress-controller

Options:

• Teach the ingress controller about oidc / auth
• Merge the ui-server and api-server

Decision:

• YAGNI (ingress-controller)
• Lets merge the 3 deployments (broker / cluster / ui)

Refactoring tasks:

• Merge all the go.mods together into a single root go.mod
• Mount the gitops-broker handlers onto the main clusters-service mux
  • There is a possibility to invest a bit more here and add the routes to the proto defs too but the multi-cluster work might make all the broker routes obsolete too.
• Add a root route to serve the ui from the clusters-service and update build processes to mount it (see how core does this)
• Remove build tooling and helm chart deployments for the ui-server and the gitops-broker

Auth tasks:

• Add auth middleware and OIDC params to clusters-service
• Add kube clientGetter impersonation to all handlers
• document how to configure OIDC in EE (maybe include adding some more values to helm-chart)