Record the reason a project got generated

[snicoll]

There are more than one reason why a project is generated but we don't have any way to differentate those at the moment. For instance, invoking Explore on start.spring.io can be used to explore an idiomatic setup or to get some information with given dependencies to update an existing project.

We don't really have a concept in the library for that and recording them as a new project generation is a bit misleading.

Perhaps an action attribute would be interesting? We should let any service implementation provide their own action with a way to extract an identifier (typically for statistics purpose).

This action should be set when generating the project (or perhaps this could be an attribute of the ProjectRequest ?