[8.18](backport #42462) Add status reporting for Journald input

[mergify]

Proposed commit message

This commit adds the status reporting for the Journald input. It also adds a debug log to the UpdateStatus function from v2.Context.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

## Disruptive User Impact
## Author's Checklist

How to test this PR locally

1. Build the Elastic-Agent with Beats from this PR or build Agent beat and replace its binary in a existing Elastic-Agent installation
2. Create a policy with "Custom Journald logs" or add the integration to an existing policy
3. Set the log level to debug
4. Deploy/Restart the Elastic-Agent
5. Look for the debug messages from the logger input.journald:
   • updating status, status: 'Starting', message: 'Starting'
   • updating status, status: 'Running', message: 'Running'
6. Ensure the Journald integration for your Elastic-Agent shows the status Healthy

Related issues

• Closes [Journald] Implement status reporting for Elastic-Agent #39791

## Use cases
## Screenshots
## Logs

---

This is an automatic backport of pull request #42462 done by [Mergify](https://mergify.com).

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)