Decide on adding radio buttons for application configuration format

[mhalbritter]

In spring-io/initializr#1682, initializr learned to create YAML configuration files.

We should decide if we want to surface that in our UI.

[mhalbritter]

We talked about that, and decided we could add radio buttons for the configuration format: either properties or YAML. properties would be the default, but like the other radio buttons, the preference should be stored in the browser local storage.

when this is implemented, we also should update the Spring Boot CLI for that feature.