REVIEW [NX-163] extender model to register plugins/bundles

https://issues.sonatype.org/browse/NX-163
https://bamboo.zion.sonatype.com/browse/NX-OSSF35-15 :white_check_mark: 

Enables use of alternative bootstrapping, such as Jetty-in-OSGi registration via ContextHandler service.

Summary of changes:
* New nexus-extender automatically registers plugin/bundles with Nexus as they're started
* Embedded nexus-core and associated support jars as part of nexus-extender bundle
* Bootstrap layer now consists of nexus-bootstrap, some logging/metric jars, and framework launcher
* Webapp bootstrap mode launches OSGi framework + Nexus from inside Jetty/Tomcat/JavaEE
* Jetty/OSGi bootstrap mode launches Nexus on top of an existing OSGi framework
* Includes example Karaf assembly which can launch Nexus (see README for instructions)