Allow to configure K8s ns and flux resource path in API

This change allows to configure the Kubernetes Namespace and Flux Resource Path in the Environments REST API.

There are more MRs in this stacked diff:

1. Expose cluster agent in Environments REST API (!164862 - merged)
2. Allow to configure cluster agent in Environment... (!164864 - merged)
3. Expose k8s ns and flux resource path in API (!164866 - merged)
4. Allow to configure K8s ns and flux resource pat... (!164886 - merged)

Refs Allow users to set Agent to an Environment via ... (#412677 - closed)