Add top-level --version flag

[fabiante]

Having an option to call java -jar pdftools.jar --version would be nice to figure out what version of the tool is installed.

Currently CLI users can not use such a command to figure out what version of this tool is installed.

[fabiante]

This may be helpful: https://picocli.info/#_dynamic_version_information

I guess that this requires somehow getting the version from the manifest / jar metadata and returning it via the documented interface.

[fabiante]

A couple of sources I skimmed through:

• https://stackoverflow.com/questions/1272648/reading-my-own-jars-manifest
• https://stackoverflow.com/questions/62963202/why-doesnt-picocli-recognize-my-option-from-the-command-line
• https://github.com/remkop/picocli/blob/main/picocli-examples/src/main/java/picocli/examples/VersionProviderDemo1.java
• DOC: improve example/docs getting version from jar manifest remkop/picocli#1663

Implementing the required interface is kind of easy:

package one.squeeze.pdftools.cli;

import picocli.CommandLine;

/**
 * Provides a version for CLI commands based on the manifests implementation version.
 */
public class ManifestVersionProvider implements CommandLine.IVersionProvider {
    @Override
    public String[] getVersion() throws Exception {
        return new String[]{
            "some version" + this.getClass().getPackage().getImplementationVersion()
        };
    }
}

My problem with this is, that using this provider on the root command does not seem to work. I don't know why.