chore: Add signing plugin and configure signing credentials

Integrates the signing plugin and sets up signing key and password as environment secrets for publishing. Ensures version resolution logic adapts to the publishing process by utilizing the new `IS_PUBLISHING` flag.

Author: xenoterracide

.github/workflows/gh-publish.yml

@@ -41,3 +41,5 @@ jobs:
         env:
           ORG_GRADLE_PROJECT_ghUsername: ${{ secrets.GITHUB_ACTOR }}
           ORG_GRADLE_PROJECT_ghPassword: ${{ secrets.GITHUB_TOKEN }}
+          ORG_GRADLE_PROJECT_signingKey: ${{ secrets.SIGNING_KEY }}
+          ORG_GRADLE_PROJECT_signingPassword: ${{ secrets.SIGNING_PASSWORD }}