Explicitly add classifier to maven publication

[tha00]

It seems that after 960617f, archive classifiers are dropped from the archive file name due to the addition of the artifact via publication.artifact(File). According to the Gradle docs

Extension and classifier values are interpolated from the file name.

but for some reason it is not happening in ShadowExtension.

This PR provides a workaround by explicitly adding the classifier using the option to pass a Map to MavenPublication#artifact:

A Map that contains a 'source' entry that can be resolved as any of the other input types, including file. This map can contain a 'classifier' and an 'extension' entry to further configure the constructed artifact.

• Closes "-all" classifier dropped from published jar #860.
• Closes Configuring publishing before shadowJar publishes jar with wrong configuration #945.

[tha00]

@Goooler, do you have any objections to merging this PR, now that the move to GradleUp is done?

[Goooler]

Will review this change carefully later.

[Goooler]

Thanks a lot!