* [JENKINS-37332] - Improve diagnostics of non-closed streams during reading of the manifests in PluginManager

* [JENKINS-37332] - Leakless processing of JarUrlConnection during Manifest parsing

* [JENKINS-37332] - Also implement leak-safe method for retrieving file modification date

* [JENKINS-37332] - Add spotcheck methods for manifest file access + Javadoc

* [JENKINS-37332] - Also test multi-line and empty attributes in the test