[MM-18150] plugin panic trace should not be lost (#13559)

* Transit panic from debug to error

* Parse plugin's StdErr and output panic to the mlog.Error

* Add unit tests

* Change log test

* Remove buffer from logger

* Remove 'panic' string filter

* Change *Buffer to io.Writer

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

mlog/stdlog.go

@@ -85,3 +85,13 @@ func (l *loggerWriter) Write(p []byte) (int, error) {
 	}
 	return len(p), nil
 }
+
+type panicLoggerWriter struct {
+	logFunc func(msg string, fields ...Field)
+}
+
+func (l *panicLoggerWriter) Write(p []byte) (int, error) {
+	trimmed := string(bytes.TrimSpace(p))
+	l.logFunc(trimmed)
+	return len(p), nil
+}