Support more data types in plugin attributes

Currently, annotating plugin factory method parameters with @PluginAttribute only supports String types, but we use a lot of plugin attributes that could be automatically converted to primitive types. Now I'm not suggesting something as advanced as how Camel handles parameter injection (which does automatic non-trivial type conversion), but it would be great to support booleans, ints, longs, etc., to simplify quite a bit of extraneous string parsing.