Add extra pages to pkg installers

[marcoesters]

Description

Summary of changes

• Add feature to create extra pages to pkg installers after the installation step, analogous to the capability for Windows installers.
• Refactor code signing for pkg installers into a class.
• Add testing for application signing.
• Expand the post-install page features to Windows.
• Expand documentation about signing pkg installers.

Detailed explanation

constructor has the capability to add additional pages for exe installer by overloading the custom_conclusion property. This is not possible for pkg installer because custom pages have a different format from conclusion pages: extra pages follow the Apple plugins format whereas conclusions are RTF or HTML.

So, a new keyword for construct.yaml is required. For pkg installer, this keyword can take multiple extra pages and add them to a pkg installer. These pages can either be compiled projects or simple, single-target projects. The latter is helpful for repositories since it would not require committing binaries.

The plugins must be signed to pass notarization just like the _conda binary. To make the code DRYer, the codesign commands are now a class just like the Windows signing tools. Caveat: get_signing_command() has a different return type because they are used differently. However, I'm happy to change the Windows return type to list and do the join operation elsewhere.

Signing has not been tested on MacOS yet. The challenge is to create self-signed certificates that must be added to a keychain. This is limited to application signing. For installers, a certificate has to be trusted to be found in the keychain. Trusting a certificate requires authentication and interaction with keychains, causing the job to be stuck. I have not found a way to circumvent this.

This concept has been expanded to Windows as well. While this can still be done via custom_conclusion, using the extra pages keyword provides a cleaner solution, especially for those who use the default conclusion page.

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[jaimergp]

Thanks!