Publish snapshots to GitHub packages

**As a** contributor
**I want** snapshot versions of fabric-chaincode-shim to be published to GitHub Packages on every change
**So that** I can use development fixes or features before a formal release is published

The [release.yml](https://github.com/hyperledger/fabric-chaincode-java/blob/main/.github/workflows/release.yml) workflow already publishes to GitHub Packages (as well as Maven Central). It would be good for only the GitHub Packages publish to be done on every [push.yml](https://github.com/hyperledger/fabric-chaincode-java/blob/main/.github/workflows/push.yml) workflow. To differentiate from release versions, the Gradle package version used for the push workflow should be of the form `x.y.z-SNAPSHOT`. This is a standard [Maven naming convention](https://maven.apache.org/guides/getting-started/index.html#what-is-a-snapshot-version).

Although it uses Maven rather than Gradle, the CI pipeline for the [fabric-gateway](https://github.com/hyperledger/fabric-gateway/blob/90bd6c4f52a8369d0380f873e3fffb92ed199240/.github/workflows/push.yml#L60-L81) project could be used as a rough guide.