feat: sandbox trampoline implementation

[wolfv]

This is the new rattler_sandbox!

It works as follows:

• users of the sandbox need to call sandbox_init before almost anything else in their applications. That will enable a special CLI handler to act as a trampoline. Most importantly, it should execute before any other threads are launched.
• the trampoline in this case means that the app can also sandbox processes when invoked with e.g. rattler-build __sandbox_trampoline__ --fs-read /foo/ ls.

The API returns a regular Command struct so that arguments can be added and things can be set like cwd, env vars, etc.

We use birdcage, so things should work on Linux and macOS, with Windows support (through AppContainer) in the works.