Provide a decentralized way of specifying YANG features available at runtime

YANG-based configuration is used by more and more SDOs, to the point where it is very useful to reuse SDO-defined snippets to configure our components.

One example is the effort behind ietf-netconf-netconf-client-server, which is super useful for configuring both our NETCONF client and server interfaces.

These interfaces unfortunately rely heavily on 'if-feature' statements, which in turn exposes a gap in our infrastructure: we have no way for application components to specify which features they support.

Add the ability for OSGi bundles to expose features they support (at runtime) similar to how they advertize packaged models. Use this ability in DOMSchemaService implementations to pass the collected features down to YANG parser and thus establish a accurate view of EffectiveModelContext.