[Improvement] skip signing for non-public publishing

### What would you like to be improved?

In the current publishing process, the content must be signed using the gradle signing plugin. While signing is not necessary for local use, conditionally skipping signing can help developers build and debug more easily with other systems. See [this discuss](https://github.com/datastrato/gravitino/pull/2446#issuecomment-1986748343) for details.

### How should we improve?

signing only when publishing to public repositories

Start with the Gradle signing plugin configuration and the publishing flow, using the linked discussion for context. Done means signing occurs only when publishing to public repositories, while local and other non-public builds can complete without signing.

Written by the indexing model from the issue text.