Add a --validate option to the jar tool

Summary

Add a --validate option to the jar tool that can be used to validate jar files.

Problem

Malformed jar files can lead to weird compilation and binary/source compatibility problems, for instance due to APIs differing across different version in a multi-release jar file.

While the jdk's jar tool does not produce malformed jar files, a third party tool might, or a jar file might be manually edited after creation.

It is currently not possible to (re-)run validation logic without updating a target jar file as well.

Solution

Add a --validate option to the jar tool, which can be used to directly run the existing validation logic on a given jar file.

Specification

  --validate             Validate the contents of the jar archive. This option
                         will validate that the API exported by a multi-release
                         jar archive is consistent across all different release
                         versions.

Furthermore, the error message when a main operation is not specified is updated as well:

-        One of options -{ctxuid} must be specified.
+        One of options -{ctxuid} or --validate must be specified.