tracing-journald: message formatter

[jhruzova]

Motivation

Enable custom formatting of log messages with access to both message content and metadata before sending them to journald, while maintaining structured field support.

Changes

• Introduced a MessageFormatter trait with a method:

  fn format_message(&self, message: &str, metadata: &Metadata<'_>) -> String;

• Added support in Layer to set a custom formatter using:

  pub fn with_message_formatter(self, formatter: impl MessageFormatter + 'static) -> Self;

• Updated EventVisitor to apply the custom formatter to the message field if provided.