Expose cluster agent in Environments REST API

This change set exposes the cluster agent 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)