Generalize the initialization of extension at application startup for all type of extension

Introduce a new Initialize action for extension handlers to initialize their extension during application startup. Defaulted to do nothing in the AbstractExtensionHandler, this same method simply call install() for JAR extension.

An ExtensionInitializer allow calling this initialize method for all installed extension, either globally or for a given extension type and/or namespace.

This initializer is called during the ApplicationStartedEvent.