Add support for "start app on login" as part of packaging

### What is the problem or limitation you are having?

Some apps (in particular server and toolbar style apps) need to add themselves to the startup items list. Briefcase should be able to produce apps that can install themselves in the startup item list

### Describe the solution you'd like

Apps should have a configuration option so that when an app is installed, it is added to the startup items for the platform - e.g.,

```
startup_item = True
```

On macOS, this will require the use of a .pkg installer (see #1781)
On Windows, it will require an MSI installer
Linux packages will result in the app being added to systemd/initd.

It might be desirable to add this in a way that the user can opt-in, rather than it being a forced requirement on installation.

### Describe alternatives you've considered

None.

### Additional context

_No response_

Start by reviewing the packaging work described in #1781 and the platform-specific installer paths for macOS, Windows, and Linux. Clarify whether startup registration is opt-in, then define completion as a configuration option that produces the appropriate startup-item behavior for each supported platform.

Written by the indexing model from the issue text.