Allow Plugin Processing Kind filtering

[treyfarmigoni]

Allow filtering in printMessage based on log4j.plugin.processor.minAllowedMessageKind

Issue details discussed here
#3380 (reply in thread)

Add an log4j.plugin.processor.minAllowedMessageKind argument to PluginProcessor and use it to filter printMessage() calls based on the provided Diagnostic.Kind
Document the issue and the property in Registering plugins in a %collapsible

Checklist

• Base your changes on 2.x branch if you are targeting Log4j 2; use main otherwise
• ./mvnw verify succeeds (the build instructions)
• Non-trivial changes contain an entry file in the src/changelog/.2.x.x directory
• Tests are provided

[treyfarmigoni]

mvn verify passing