Add Kotlin code starters

[janpk]

Added Kotlin starter code for the app and the airline bot. Tests added based on the code starter tests for Java. Also tested for expected generated Kotlin output with

mvn io.quarkus:quarkus-maven-plugin:3.30.0:create \
    -DplatformVersion=3.30.0 \
    -DprojectGroupId=org.acme \
    -DprojectArtifactId=my-github-app \
    -Dextensions="io.quarkiverse.githubapp:quarkus-github-app:999-SNAPSHOT,io.quarkiverse.githubapp:quarkus-github-app-command-airline:999-SNAPSHOT,kotlin"

and that the application starts with quarkus:dev

[gsmet]

Thanks, very nice!