Build the fat-jar when a new release is published

[julienrf]

• Add a GitHub workflow that builds the jat-jar and uploads it as a release asset each time a new release is published.
• Change the assembled jar name to scylla-migrator-assembly.jar (remove the “constant version number” 0.0.1). Users can download a specific release from the GitHub releases page, if they want to.
• Update the instructions in the README to download the pre-built fat-jar instead of building it manually (I kept the instructions to build it manually, but I moved them to the bottom of the README).
• Update the Ansible recipe and the testing infrastructure accordingly
• Fix path to the assembly in the Ansible scripts along the way

Fixes #154

[julienrf]

Open question: we could rename the assembly to scylla-migrator-bundle.jar if that makes sense.

[tarzanek]

let's get git version inside fat jar (commit hash, ev. tag once we start tagging), I won't block merging of this PR due to that https://github.com/sbt/sbt-git seems to be one way to solve