Create an action to build the release artifacts

[jeremyprime]

Is your feature request related to a problem? Please describe.

Currently a developer must build the release artifacts themselves. It would be better to have a GitHub action to do this, ensuring a consistent build.

There is the ability to download the slim JAR from some existing GitHub actions, like the main action, but we probably don't want to build the shaded JAR at the same time due to it's large size (>100MB). So a separate action to run at release time, or on-demand, would be better.

Describe the solution you'd like

Create a new GitHub action (e.g. Release Artifacts) which can be run when a new release is created. That way the action can be run and the artifacts can be added to each release.