Add code signing for Linux release artefacts

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

Code signing is an option provided by the `briefcase package` command. However, Linux release artefacts don't currently use these options.

### Describe the solution you'd like

`briefcase package linux` should provide options to sign release artefacts.

`briefcase package linux --identity` should provide options to explicitly provide a signing identity.

### Describe alternatives you've considered

Do nothing. Lack of signing doesn't appear to *prevent* any apps from being installed; however, as a policy, some users might require signed artefacts.

### Additional context

It's not clear what is required to make signing work. It's quite probable that completely different approaches will be needed for each system package format, plus AppImage and Flatpak.

Start at the `briefcase package linux` command and trace how its signing options are handled. Compare the requirements for each named Linux release format—system packages, AppImage, and Flatpak—and determine what “done” means for explicit `--identity` support and signed artefacts across them.

Written by the indexing model from the issue text.