[FIXED JENKINS-37332] - Prevent File descriptor leaks when reading manifests from JARs (#2516)

* [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

(cherry picked from commit 96c97860)