feat(journald source): improve error handling for journald source by spawn new stderr handler

[titaneric]

Summary

This PR intend to spawn a new stderr handler for journald, to emit the warning when there are any error due to permisson issue etc.

Vector configuration

data_dir: vector-data
api:
  enabled: true
  graphql: true
  playground: false
  address: "127.0.0.1:8686"
sources:
  journald_logs:
    type: journald
    since_now: true
sinks:
  my_sink_id:
    type: console
    encoding:
      codec: raw_message
    inputs:
      - journald_logs

How did you test this PR?

Run the vector with the user didn't join to systemd-journal group

sudo -u vector vector -c config.yaml -vv

AS IS

TO BE

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

Closes: #23829

Notes

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