Automatic release to Maven central

[pietrygamat]

Currently there is no straight-forward way of doing deployment to Maven Central as it involves several manual steps when setting up the environment:

• setting up right Java version to ensure the project is usable for the majority of end users
• setting up published PGP key required for signing artifacts before they are accepted by Maven Central
• setting up OSSRH credentials for actual artifact deployment and release

The idea behind this PR is to automate the process using Github actions and to take advantage of the repository secrets to store credentials and keys. I know the project already uses travis-ci, and it would be quite straight-forward to achieve the same goals using this tool, but moving to github actions has the benefit of keeping critical parts of the flow reusable by forks and simplifies access management.

What this change does NOT do, but perhaps we want another PR to cover:

• Ensure the native libraries are build using the same revision for all plaforms
• Update versions in pom.xml
• Tag the exact revision sent to central
• Automatically perform release to Github Releases

If this is to be accepted, the repository maintainer needs to create 4 repository secrets referenced in the workflow file. After that deploying to Maven Central is the question of pressing a button under Actions tab.

[pietrygamat]

This will create settings.xml file. See https://github.com/actions/setup-java/blob/main/docs/advanced-usage.md#Publishing-using-Apache-Maven

[pietrygamat]

More details: https://central.sonatype.org/publish/publish-maven/

[tresf]

I know the project already uses travis-ci,

It did yes, but it's broken. Travis-Ci.org switched to Travis-CI.com completely breaking our project, so it's browsable and enabled but not firing since some time back in April.

[tresf]

Ensure the native libraries are build using the same revision for all platforms

We don't necessarily do this today, which is why we offer a native lib version information. Automating building is pretty straightforward, if we decide to tackle this in the future. Solaris will be a bit of a pain, but we have the manual steps documented in the wiki.

[tresf]

same revision for all platforms

If we automate this, I think this warrants a command line flag to overrides the version.