[Ingest Manager] Improve agent detail page to help user diagnose problems

[hbharding]

The current agent detail page was done quickly without much design involvement. The activity log table feels complicated and the messages are difficult to read. This page should be able to paint a clear picture of the agent's current status and recent history of changes, especially in the event that there is an error so we can help the user diagnose the problem.

The table layout might not be the best way to communicate all the events we report. Instead, we could show a timeline of events with the new EuiCommentList and format the messaging so the events are easier to read and understand.

Side note: The Endpoint Team has expressed interest in being able to see an audit log for agent / agent configuration changes (#66090). This might be an appropriate place to show some of this information.

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[hbharding]

Duplicate issue. Will be fixed by #81872