Switch CD to GitHub Actions

[simonbasle]

This commit switches the CD to GitHub Actions, with a few adjustments:

• CI is now only run on pull requests (ci.yml)
• Pushes to master trigger the snapshot.yml worflow which runs tests
  in one job then publishes to Artifactory /libs-snapshot-local in a
  second job
• Publishing a GitHub Release or PreRelease triggers release.yml
  • the tag is used as the version, so it must end with .RELEASE or
    represent a pre-release and include .RC or .M (this is added as a
    condition to the publishing jobs)
  • tests are not run, as it is assumed that snapshot tests have been
    verified by the maintainer who triggers the release
  • prereleases go to Artifactory /libs-milestone-local
  • GA releases still go to Bintray for now

Segregation of deployments into two jobs allow to use an environment and
thus properly gate access to secrets. It also gives an opportunity to
avoid all 3rd party actions in the critical parts of the workflow.

Fixes #159.

[simonbasle]

Some caveats of the previous CI/CD and release process listed while working on this PR:

• the version is NOT defined in gradle build files but rather defined at CD time in the script(s)
• tags are made without a v prefix (important, as they're used as the version for a given release)
• doesn't follow the calver scheme (important, as .RELEASE suffix is used to determine a release vs a milestone vs a snapshot)
• publish doesn't trigger tests
• tests must be run with JDK10+ due to examples
• snapshoting on azure pipelines run with JDK11 configured for Gradle, but the target and source versions are still Java 8 anyway

[bsideup]

added a minor clarification

[simonbasle]

The snapshot environment has been prepared, so merging this should trigger a snapshot deployment.