Summary:
The field enable is removed from NodeAgentEnabler as it is always enabled. yb.node_agent.client.enabled is made INTERNAL before it will be removed in later releases. Node agent mandatory check is added for Live nodes. After the
migration is complete, node agent is installed on new nodes during tasks like add node or edit universe.

Test Plan: Manually tested by unregistering node agent and running edit U to add a node. It failed. After installing node-agent, it succeeded.

Reviewers: svarshney, anijhawan, nbhatia

Reviewed By: svarshney

Subscribers: yugaware

Differential Revision: https://phorge.dev.yugabyte.com/D46172