Plugins support (outputs)

The generic events support already handled most of this, with a
dedicated formatter factory for plugin sources. Just one missing
header include and change the logic slightly for json parsing.

Co-authored-by: Leonardo Grasso <me@leonardograsso.com>
Co-authored-by: Loris Degioanni <loris@sysdig.com>
Signed-off-by: Mark Stemm <mark.stemm@gmail.com>

userspace/engine/formats.h

@@ -17,6 +17,7 @@ limitations under the License.
 #pragma once
 
 #include <string>
+#include <map>
 
 extern "C"
 {