XPC Service Integration Changes

[zorgiepoo]

After #1812 (a necessary bit) and #1813 lands, I want to change guidance on our sandboxing docs to make the org.sparkle-project.InstallerStatus.xpc and org.sparkle-project.InstallerConnection.xpc services optional, and instead encourage developers to adopt these entitlements to their application:

<key>com.apple.security.temporary-exception.mach-lookup.global-name</key>
<array>
    <string>$(PRODUCT_BUNDLE_IDENTIFIER)-spks</string>
    <string>$(PRODUCT_BUNDLE_IDENTIFIER)-spki</string>
</array>

The temporary bit in the entitlement is in context to being approved in the App Store but that's not a problem for Sparkle applications.

I think this is a better default approach than having these services spawn to communicate to the installer or progress agent tool.

In an unusual constrained environment where sandbox entitlements cannot be changed (eg due a plug-in inheriting a sandbox), the services may still be useful to provide as an option however. Also they might be useful to keep for developers that won't read updated instructions and accidentally break their app.