Add boilerplate repo for Kotlin backend

[mialbu]

Goal

Neow3j has two boilerplate repositories. One for contract development and one for using neow3j's SDK. While both are Java projects, neow3j's SDK also supports development for Kotlin backend. Thus, it would be great to have a neow3j boilerplate repository to help developers get kick-started with Kotlin backend development.

Approach

We might want to have this boilerplate in a separate repository and if so only create it once it's actively being worked on, i.e., actively being created. If someone is interested in taking this issue and creating this boilerplate for Kotlin backend, please add a comment to this issue mentioning @mialbu, @csmuller or/and @gsmachado, so that we can create the boilerplate repository, and then you can fork it and create a pull request with the boilerplate code from your fork.