[signingscript] Refactor behaviors and allow taking in all files at once

[hneiva]

In #957 we bypass task.sign for apple_notarization_stacked behaviour for all-at-once notarization, we should refactor how we handle the list of files.

Entrypoint that needs adjusting in signingscript/src/signingscript/script.py -> async_main, which calls signingscript/src/signingscript/task.py -> sign and then calls each behaviour. This could either mean all behaviours should also take in a list of files to iterate over instead of a single file, or some way of dynamically deciding when a behaviour should or should not take in all files at once.

[jcristau]

@hneiva can you retitle this issue? title seems incomplete :)