Add PGP signing to publications

[lamba92]

Introduce Pretty Good Privacy (PGP) signing to the publication workflows of the project. The signing setup is included in the gradle wiring along with the retrieval of the key details from environment variables. This change is applied in both the publish and hotfix workflows.

[rock3r]

@lamba92 build fails with

FAILURE: Build failed with an exception.

* Where:
Build file '/home/runner/work/jewel/jewel/decorated-window/build.gradle.kts' line: 3

* What went wrong:
An exception occurred applying plugin request [id: 'jewel-publish']
> Failed to apply plugin 'jewel-publish'.
   > null cannot be cast to non-null type kotlin.String