Replace the flat-parameter log(level:message:metadata:source:file:function:line:) method on LogHandler with
log(event: LogEvent), enabling forward-compatible evolution of the LogHandler interface without breaking existing
handler implementations.

Motivation:

Changes to the LogHandler API require dance around creating new overloads, deprecating old overloads, which unnecessary bloats LogHandler protocol surface (like happened with the source: parameter). We have upcoming changes that would introduce new capabilities to the LogHandler protocol, for example https://forums.swift.org/t/proposal-slg-0003-standardized-error-metadata-via-logger-convenience/84518/35. It makes sense to introduce a single container structure for all the data associated with a log event before introducing any new overloads in LogHandler protocol.

Modifications:

• New struct LogEvent is added.
• LogHandler protocol now has an additional log(event: LogEvent) method.
• Logger is calling handler's log(event: LogEvent) method.

Result:

Further changes to the LogEvent with new information passed from the Logger to a LogHandler does not require changes in the LogHandler API. As a bonus, LogEvent makes source a computable variable, improving performance for log handlers who do not need the default source created based on the log file location.