Parallelise calls to app signing

[freakboy3742]

What is the problem or limitation you are having?

When signing a macOS app, briefcase needs to sign each individual .so file. This is currently being done sequentially, which can take a while.

Describe the solution you'd like

Briefcase should parallelise the calls to sign individual files in the bundle, with the degree of parallelisation reflecting the number of CPUs available on the machine.

Describe alternatives you've considered

Do nothing. Single threading works; it's just slower.

Additional context

ThreadPoolExecutor should be a fairly approachable way to implement this.

[xairos]

I'd like to sprint on this!

[nondescryptid]

Hi! I currently have a draft PR for this. It's a draft as I noticed that there are comments in the sign_app() function at src > briefcase > platforms > macOS > init.py that mention:

# Signs code objects in reversed lexicographic order to ensure nesting order is respected
# (objects must be signed from the inside out)

I am not sure of how strict this rule is, and what dependencies/other things rely on this rule being followed. If it is a strict requirement and lots of things might break because of not signing files in order, writing files sequentially may be the preferred implementation even if it is slower.
I tested my parallelised implementation with the BeeWare helloworld tutorial, which only uses Toga. The parallelised implementation works on the helloworld tutorial app, but I'm not sure if it'll still work on apps that use more than just Toga.