Make Stateful Functions release-ready

As a preparation for releasing the first release for Apache Flink Stateful Functions, there's a few things that needs to be done:

Legal

• Missing NOTICE files for canonical source distribution in project root
• Missing NOTICE / LICENSE files under META-INF directory of to-be-released Maven artifact jars.

Tooling
Need utility scripts to create release branches, create source distribution, building and deploying staging artifacts, etc. Most of this can probably be based on existing tooling in apache/flink.

Stateful Functions releases would also include a base Docker image for building applications, so we also need to look into how we want to do that. Most likely this will be blocked by the ongoing efforts in releasing an official Flink image endorsed by the Apache Flink PMC.

Documentation
All information about releasing a Stateful Functions release should also be documented in the Flink community project wiki.

This is an umbrella issue covering all necessary subtasks.