Migrate Jenkins release job to GitHub Actions.

[rgrunber]

• Preserve existing Jenkinsfile as a fallback

I've been sitting on this for a while, but I believe it should be working as expected now. It contains a few improvements over the existing Jenkinsfile workflow. Of note :

• A new option that allows the packaging of the extension by downloading the exact JDT-LS tarball from https://download.eclipse.org/jdtls/milestones/
• Exposing of the candidate vsix packages prior to publishing via. https://github.com/redhat-developer/vscode-java/releases (draft release)