‼️ Should be reverted if elastic/elastic-agent#4747 does not make 8.15.0.

Summary

Resolves #180778

This PR allows agent log level to be reset back to the level set on its policy (or if not set, simply the default agent level, see elastic/elastic-agent#3090).

To achieve this, this PR:

• Allows null to be passed for the log level settings action, i.e.:

POST kbn:/api/fleet/agents/<AGENT_ID>/actions
{"action":{"type":"SETTINGS","data":{"log_level": null}}}

• Enables the agent policy log level setting implemented in [Fleet] Expose agent logging level in agent policy settings #180607
• Always show Apply changes on the agent details > Logs tab
• For agents >= 8.15.0, always show Reset to policy on the agent details > Logs tab
• Ensures both buttons are disabled if user does not have access to write to agents

Caveats

1. The reported agent log level is not accurate if agent is using the level from its policy and does not have a log level set on its own level (log_level metadata is not accurate elastic-agent#4747), so the initial selection on the agent log level could be wrong
2. We have no way to tell where the log level came from (log_level source should be reported elastic-agent#4748), so that's why Apply changes and Reset to policy are always shown

Testing

Use the latest 8.15.0-SNAPSHOT for agents or fleet server to test this change

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Unit or functional tests were updated or added to match the most common scenarios