Add support for post install scripts on macOS PKG installers

[freakboy3742]

What is the problem or limitation you are having?

Originally reported by @haikesan as #2561.

MSI installers currently support the inclusion of post install scripts.

PKG installers also have the facility for post install scripts; however, Briefcase will not currently make any use of them.

Describe the solution you'd like

It should be possible to add:

post_install_script = "scripts/post_install.sh"

to a macOS project configuration, and have that project configuration include the named script as part of a PKG installation.

Describe alternatives you've considered

n/a

Additional context

There's no analog of a pre-uninstall script for PKG, so the user should be warned if they define a pre-uninstall script for a PKG installer. They should also be warned if they produce a DMG or ZIP installer and have pre-uninstall or post-install scripts defined.

[haikesan]

@freakboy3742 I've implemented this concrete feature. In original issue, I've mentioned "include custom payload for installer-only". Since my implementation builds pkg_scripts/, I can also copy custom payload there, however, this is out of scope of THIS particular issue, and in scope of #2561. Can I implement this feature also, as some post_install_resources field?

Edit: Intended behavior is already achievable, because post-install runs after unpacking. So we can run "/Library/My App/My App.app/Contents/Resources/helper" --options. But these are not installer-only resources

[freakboy3742]

It wasn't clear to me from the conversation on #2561 that you had a need for resources above and beyond what was part of the "normal" bundled app. Since macOS PKG installers have the capacity to carry resources, I agree it would make sense to include that capability.

I'd suggest a simpler installer_resources setting, as there's only a single installer artefact. The user-supplied value should be a single directory; at build time, the build process creates a final_resources folder that copies everything from the templated installer/resources folder, plus everything in the user-supplied installer_resources folder. The productbuild invocation then points at final_resources as the --resources value. This ensures that user resources are included, but any "stale" resources from a previous build are not included in future PKG builds.

Does that make sense to you? If it does, I'd be happy to have it included in your PR for #2903.