[Bug]: Unity 6.3 Thinks the package is missing a signature and is marking this as unsafe

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Description

Unity 6.3 introduced signed packages. We need to make this signed somehow.

Steps to Reproduce

1. Open UPM

Expected Behavior

Package looks normal

Actual Behavior

There is a large warning sign on the package

Unity Version

6000.x

Package Version

3.1.6

Platform

• Editor
• Windows
• macOS
• Linux
• iOS
• Android
• WebGL
• Other

Additional Context

This needs to be backwards compatible, work with Open UPM, NPM, and ideally git urls

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More details here:

https://docs.unity3d.com/6000.3/Documentation/Manual/upm-signature.html

https://docs.unity3d.com/6000.3/Documentation/Manual/cus-export.html