[Standalone manifest] use hostPath to store elastic-agent state

[tetianakravchenko]

In #2550 was introduced hostPath to store elastic-agent state for both standalone and managed elastic-agent manifests, main reason for that change was to aling behaviour with the ECK and comply with the documentation: agent.ephemeral_id normally changes across restarts, but agent.id does not.

But for the standalone manifest STATE_PATH variable is defined - https://github.com/elastic/elastic-agent/blob/main/deploy/kubernetes/elastic-agent-standalone/elastic-agent-standalone-daemonset.yaml#L64C21-L65, so the hostPath is actually not used.

STATE_PATH was introduces to support hints - #1202 (some discussion about this: #663)

We should: make use of the hostPath to store the agent state, at the same make sure that hints functionality is working.

Related issue: #953

[tetianakravchenko]

• PR Kubernetes standalone manifest: remove STATE_PATH, replace initcontainer image #4625
• documentation Update hints autodiscovery documentation ingest-docs#1050