Reviving discovery of CRaSHPlugins via Spring

I noticed `SpringBootstrap` used to have code to obtain Spring beans of type `CRaSHPlugin`; however, the resulting list wasn't actually used and can understand why the code was removed at commit 064f6f131bc256d33f66d71f911e6477ff1c7ed5. 

This seemed like a very useful feature since our app is heavily Spring based. It appeared extending `ServiceLoaderDiscovery` with the Spring logic made more sense and that did indeed work for our application.