chore: add sign target to gradle builds

[acke]

Description

These steps are needed to sign plugins:

1. Generate the certificate. https://plugins.jetbrains.com/docs/intellij/plugin-signing.html#plugin-signature-verification
2. Add the certificate to Github secrets.
3. Implement the signPlugin task for Gradle.

This PR covers the implementation for step 3.

Checklist

• [] Tests added and all succeed
• [ ] Linted
• [ ] CHANGELOG.md updated
• [ ] README.md updated, if user-facing

Screenshots / GIFs

Visuals that may help the reviewer. Please add screenshots for any UI change. GIFs are most welcome!