Support custom log formats for apps

[agners]

Description

Some apps don't print date/time in their logs (e.g. Let's Encrypt, this PR enables it for users cutesy home-assistant/addons#4754). systemd-journald stores the date of every log entry already. We can optionally serve logs to the frontend with date prepended (we already do something similar for host logs).

Additional context

No response

[agners]

The feature is also useful for OpenThreaed Border Router which currently uses relative timestamps. Using the underlying log systems timestamp is preferred solution from upstream perspective (see openthread/ot-br-posix#3500).