CI/CD: sign using Jetbrains marketplace signer instead

[AgiMaulana-Justika]

Gradle signer causes

java.lang.NullPointerException: pemObject must not be null

https://github.com/AgiMaulana/FlutterTestFileCreatorIntellijPlugin/runs/6948621993?check_suite_focus=true

The current release signed with JetBrains/marketplace-zip-signer on local machine manually. Would be nice if we move this to Github Action.

Ref:
https://plugins.jetbrains.com/docs/intellij/plugin-signing.html#cli-tool

[AgiMaulana]

resolved in #16