Add the ability to define app permissions

[freakboy3742]

Adds the ability for an app to declare permissions, as well as some related extension points.

Adds initial cross-platform permissions for camera, microphone, fine_location, coarse_location, background_location, and photo_library. This is done by adding entries to pyproject.toml like:

permission.camera = "You can use the camera to take photos of cats".

The cross-platform definitions are converted into platform-specific interpretations:

• iOS - adds keys to the Info.plist
• macOS - adds keys to the Info.plist and Entitlements.plist
• Android - adds <uses-permission> and <uses-feature> content to AndroidManifest.xml
• Flatpak - adds finish-args to the manifest.yml

In addition, the platform-specific mechanisms (Info.plist etc) have been exposed so that apps can define platform-specific modifications and extensions that aren't covered by the cross-platform capabilities.

• iOS - info definitions
• macOS - info and entitlement definitions
• Android - permission and feature definitions, plus extension points thats allow injection of arbitrary content into AndroidManifest.xml
• Flatpak - finish_arg definitions, plus modules_extra_content to allow injection of arbitrary build rules into the Flatpak manifest.

Requires the use of updated templates:

• iOS: Add extension points for permissions. briefcase-iOS-Xcode-template#30
• Android: Add extension points for permissions. briefcase-android-gradle-template#79
• macOS app: Add extension points for permissions. briefcase-macOS-app-template#35
• macOS Xcode: Add extension points for permissions. briefcase-macOS-Xcode-template#35
• Flatpak: Add extension points for permissions. briefcase-linux-flatpak-template#33

The tests (including app build tests) all pass because the extra template context generated by Briefcase is ignored if the template doesn't support the key. The template PRs are all based on this PR, so they're exercising the new template variables - although they're not actually doing anything with them (other than the defaults injected on macOS, Android and Flatpak)

Fixes #547.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct