Improve support for bootstrapping a development environment without an active network connection

When you run a briefcase command for the first time, it requires an active network connection, and will download multiple gigabytes of tools. This is particularly bad on macOS/iOS (which require Xcode) and Android (which needs the sdkmanager an associated tools). This is a problem in bandwidth-limited scenarios (like tutorial sessions or conference wifi). It can also be a problem in environments where network access is limited or compromised (e.g., behind aggressive corporate firewalls)

It should be possible to give someone a USB drive populated with files, and bootstrap their briefcase installation.

**Describe the solution you'd like**

A new command:
`briefcase bootstrap `

will inspect ``, and install all the tools needed for ``. This may not 100% remove the need for network activity, but it should be possible to dramatically lower the initial bandwidth/time requirement.

A parallel command/option will also be required to *create* the archive folder in the first place (`briefcase bootstrap --create `?)

**Describe alternatives you've considered**

No real options, other than requiring an active network connection.

**Additional context**

Support packages and templates can be copied directly from the source folder; it may be desirable to require them to be stored in zip format.

The Android sdkmanager has support for a `--package_file` argument that can be used to directly install packages from a zip file. It's not clear

There is likely not much we can do about Xcode, as it can't be distributed outside the App Store.