Sign our release packages

(Originally reported here: https://community.cesium.com/t/webgpu-cesium-unity-1-20-0-observations-comments-possible-bugs/44849/6)

Starting in 6.3, Unity shows a warning for packages that are not signed:

So we should probably sign our packages.

More details about this system here:
https://docs.unity3d.com/6000.3/Documentation/Manual/upm-signature.html

And instructions for signing a package are here:
https://docs.unity3d.com/6000.3/Documentation/Manual/cus-export.html

I don't love that command-line signing apparently requires passing a username/password on the command-line. 🙄 Maybe we can set up a dedicated user in our Unity organization for this.

Start with Unity's package-signature documentation linked in the issue, especially the signing and export instructions. Trace the repository's release-package workflow, if present, and determine how credentials can be supplied safely; done means released packages are signed and Unity 6.3 no longer warns that they are unsigned.

Written by the indexing model from the issue text.