Add a way for applications to customize the `InitService`

### What needs to get done

The `InitService` should provide a clean way for applications to:
1. Validate the project name and other variables that will go into the template
2. Emit a message unique to each template

### Why it needs to get done

1. If the user provides an invalid name, the project will fail to build
2. Rockcraft provides different documentation links, depending on the template

Start by locating the InitService implementation and its existing template-generation tests. Trace how project variables and template-specific output are currently handled, then identify the extension point needed for validation and per-template messages. Done means applications can customize both behaviors and the relevant tests cover them.

Written by the indexing model from the issue text.