Add support for Custom URI Schemes

**Is your feature request related to a problem? Please describe.**
Some applications might want to register a custom URI on the system, for example when you go to `example://thing` it can launch the app. This may be useful when you want to launch an app from a web page. As Custom URIs support parameters, you can even pass query strings like normal URLs.

**Describe the solution you'd like**
As far I know, all operating systems are already supports custom URI handlers. Some of them are:
* Android - with modifying the manifest (Android calls as [Deep Linking](https://developer.android.com/training/app-links/deep-linking))
* Windows - [by modifying the Registry](https://docs.microsoft.com/en-us/windows/win32/search/-search-3x-wds-ph-install-registration#installing-and-registering-a-protocol-handler)
* Linux - by adding `MimeType=x-scheme-handler/example;` entry to `.desktop` file

As beeware's own cookiecutter templates contains files for setting build configuration, a `uri-schemes` option can be added to `[tool.briefcase]` setting, then inject scheme handler with cookiecutter templates.

```toml
[tool.briefcase.app.example]
formal_name = "Example"
description = "An example app."
author = "Yusuf Cihan"
uri-schemes = [
"example" # Will register "example://" to system.
]
```

I'm considering creating a PR for this feature, but I still wanted to create this issue, as someone might be able to add this feature faster than I can.

**Describe alternatives you've considered**
I'm currently cloning the cookiecutter templates from beeware's own repositories and replacing .wxs and .desktop files with my modified files, however if briefcase had a configuration itself that adds required keys to build files, it would be very easy.