Enable DRL codegen by default, rename config properties

enable model generation by default.

contextually rename properties as follows:

• `kogito.codegen.rules` : boolean – enable codegen for rules. Defaults to null, meaning that codegen is enabled iff DRLs are found
• `kogito.codegen.processes` : boolean – enable codegen for processes. Defaults to null, meaning that codegen is enabled iff BPMNs are founds
• `kogito.sources.keep` : boolean – keep DRL files in the generated artifact. Defaults to false.
• `kogito.di.enabled` : boolean – generate dependency injection annotation. Defaults to true.