Convert MWExceptionHandler to use structured logging

Convert MWExceptionHandler to use structured logging

Replace wfDebugLog() calls in MWExceptionHandler with direct use of
LoggerFactory and LoggerInterface. Logged exceptions are added to the
log message context.

LegacyLogger is also updated to append stack traces to any log event
when $wgLogExceptionBacktrace is true and the PSR-3 recommendation of
passing the exception as an 'exception' context item.

Handling of context data in LegacyLogger is expanded to support arrays,
exceptions and common object types.

Bug: T88649
Change-Id: I71499d895582bdea033a2516c902e23e38084080