ConfigContextResolver provides a way for a plugin to specify the context (ItemGroup) used to retrieve
configuration files from for classes deriving from ItemGroup it defines.
handles a request like `http://localhost:8080/jenkins/job/myjob/configfiles/show?id=MY_FILE_ID`
to allow the preview of a config file with the ID 'MY_FILE_ID' from a config screen.
A ConfigProvider represents a configuration file (such as Maven's settings.xml) where the user can choose its actual content among several concrete contents that are
pre-configured.