Split of artifact-manager-jclouds plugin

[jglick]

Split the generic parts of this plugin into https://github.com/jglick/artifact-manager-jclouds-plugin. While JenkinsRule-based tests pass, it does not work in an actual Jenkins server due to class loading conflicts. artifact-manager-jclouds must use Mask-Classes so it can load its own copy of Guava compatible with jclouds, rather than the ancient copy bundled in Jenkins core. But then artifact-manager-s3 is stuck: if it also masks the same classes, it will not agree on Guava types being used to communicate between the two (identical bytecode would be loaded twice); if it does not, then it picks up the old copy from core, which does not work. Plugin-First-ClassLoader would not help here. Global-Mask-Classes probably would—this would allow artifact-manager-jclouds to ignore core’s version, and artifact-manager-s3 to also ignore core’s version but inherit from artifact-manager-jclouds—but this would be extremely risky, as it would mean that any plugin in Jenkins attempting to refer to Guava would fail. The Jenkins plugin metadata simply offers no way to express that some classes should be masked only from core but not from plugin dependencies. So the only apparent ways forward would be:

• Add such a system to core and begin requiring that baseline.
• Partially revert [JENKINS-50520] Stop shading jclouds, update to 2.1.0, fix Guice & Guava issues #44, shading Guava + Guice + jclouds.

[jglick]

No immediate plans to resume work on this. The one other known ArtifactManagerFactory implementation, for Azure, does not use jclouds anyway.

[jglick]

See JENKINS-56795 for the GUI.

[jglick]

the ancient copy bundled in Jenkins core

See JENKINS-36779.