Create build and release infrastructure

We should implement a build and release infrastructure based on GitHub [actions](https://github.com/features/actions).
The repo has three artefacts that need to be published to [sonatype](https://central.sonatype.org):
- The shadow jar
- The docs jar
- The sources jar
All three artefacts are produced by the Gradle build.

We should have two workflows:
- One that publishes the artefacts to sonatype (also building the code and running tests). Will be run manually on main branch.
- One that runs automatically on each PR branch (builds the code and runs tests).

As an example we can look at [s3mock](https://github.com/adobe/S3Mock).