Create BOM for components included into the Core

The idea of this BOM is to simplify the dependency management in plugins and modules.

Proposed changes:

1. New BOM module in the jenkinsci/jenkins repository. We could have an external BOM, but it complicates the release management
2. Move library definitions from Parent POM and Jenkins WAR to the BOM file
3. Update WAR and Core components to use BOM
4. Update plugin POM to optionally include BOM if possible (depending on jenkins.version)