Issue 466: Expose plugins for HttpInterceptorPipeline

[mikkokar]

Description

This PR tweaks plugins loading so that the plugins will be available for InterceptorPipeline object.

Specifically, when started as a stand-alone application:

• plugins.active server configuration attribute is now optional. It can be omitted when styx is started with the routing object engine.
• httpPipeline attribute activates the new routing object engine over the current application router. In this case all plugins are loaded from plugins.all section.

When started programmatically:

• Can be started only in the old application router mode.
• All plugin factories passed to StyxServerComponents.Builder.pluginFactories() are loaded into the pipeline, in the order of appearance.