Promote to_apple_arch in conan.tools.apple

[jcar87]

Changelog: Feature: Promote to_apple_arch in the new conan.tools.apple module.
Docs: conan-io/docs#2722

Closes #11865

Notes:

• Where possible, have replaced uses of conan.tools.apple.apple.to_apple_arch to the new one taking a conanfile as an argument
• Elsewhere with this wasn't possible (because there's no conanfile object in the context), have made those call the private implementation.
• Have added a barebones test to cover this

[memsharded]

I am slightly concerned about some recent tools similar to this, as I am wondering what will happen if at some point users want to use it for the build context. They will not be able. So the options would be:

• To expose the underlying dict, something like apple_archs(arch)
• To create a new to_apple_arch_build(conanfile) (a bit ugly?)

I am fine with this change at this moment, but something to have in mind and maybe discuss.

[jcar87]

That's a good point! If there's a risk that we may end up with too many small tool functions that have a similar issue, I'd be more inclined to try and sort this out sooner rather than later. to_apple_arch has very few uses in Conan Center at the moment, so we may have an opportunity to get it right to cover all cases (although we also want to avoid a repeat of the situations where recipes are using it from the "private" module apple.apple)

[SSE4]

there are some other alternatives, like:

• change context in scope via RAII, e.g. with context_switch(ctx="build"): do something
• provide a nice moniker like if is_apple_arch(conanfile.build) where conanfile.build is ConanFIle instance with context switched to build

[memsharded]

Related #11926

[memsharded]

We will move forward with this approach, and add a context_build=False argument in the future if need to apply to the build context.