Native Image support

[sbaier1]

Problem or use case

By leveraging SubstrateVMs native image feature, we can package the MQTT CLI into a single binary, removing the immediate dependency on a local Java installation and greatly improve the startup time of the CLI, making scripted and non-shell usage more attractive.

The existing implementation of this feature (current state is at #140 ) is unstable and not ready for release.

Preferred solution or suggestions

• Improve the configuration of the already included native image task
• Fix known issues
• Add tests to CI that verify that the native image is operational after build
  • build native image in travis build
  • execute tests (shell script based? simple unit test? unclear what technology to use here yet. needs to be able to test interactive shell mode!)
• Update to the latest GraalVM version
• Release native image for linux, darwin and windows with each release

Known Issues

• TLS does not work Single-binary ECC (ECDSA/ECDHE) TLS support for SubstrateVM oracle/graal#1336 -> fixed in GraalVM 19.3 and up
• hitting Ctrl+D in shell mode yields org.jline.reader.EndOfFileException -> fixed by handling exception properly
• Commands such as help pub in shell mode do not behave as expected: Unmatched argument at index 1: 'pub'
• CLI always starts in trace mode (might be due to a missing IncludeResources flag? unsure)

[sbaier1]

updated to GraalVM 20.1.0, remaining check points still valid unfortunately.

[ghost]

Support was added in PR: #311
Tests and fixes were added in PR: #308
For now we do not have a release process for the native-image yet