Add ability for user-level configuration options #2279

What is the problem or limitation you are having?

At present, the only mode of Briefcase configuration is "project level" - if a setting isn't in the pyproject.toml, it must be specified manually at the command line of any given Briefcase command.

However, there are some values that will be consistent on a per-user basis for a project, or across all projects. There should be a way to configure project per-user and global per-user options for Briefcase.

For example, settings that you may want to define as defaults, but not persist in a pyproject.toml may include

• Your preferred iOS/Android simulator
• The credential identifiers to use when signing a project
• Cache or proxy configurations

Describe the solution you'd like

A briefcase config command that allows a setting to be defined:

• briefcase config iOS.device "iPhone 16e"
• briefcase config macOS.identity "...."

These configuration items would be saved in a .briefcase/config.toml file in the project directory; .briefcase should be added to the .gitignore for a new repository.

briefcase config --global option that would save the same options on a global basis. Global configuration items would be saved in a config.toml stored in a platform-appropriate configuration location (i.e., PlatformDirs("org.beeware.briefcase", "BeeWare").user_config_dir / "config.toml").

Global configuration options should take priority over user-level project options, and project level options should take priority over user-level project options. (i.e., if you configure an "iPhone 14" global option, "iPhone 15" as a user-level project option, "iPhone 15" would take priority; if you specify "iPhone 16" at the command line, "iPhone 16" would take priority).

The configuration structure for configuration options should match pyproject.toml - so there should be possible to configure options at a global, macOS, and macOS.Xcode level, with the same prioritization as they have in pyproject.toml.

Any option that has a command line option and can be set as a default should also allow for a ? value - so, even if I configure a default simulator of iPhone 15, if I run briefcase run iOS --device ?, the command should override the configured option and ask.

Describe alternatives you've considered

Do nothing, and continue to require all options to be explicit.

Additional context

As a first pass, I'd suggest the only options that should be exposed are:

• Author name (used in the new wizard)
• Email (used in the new wizard)
• simulator choices
• signing identities