Closes #6356: Install built-in extensions using GV extension controller

[csadilek]

This is replacing the deprecated runtime.registerWebExtension with the new webExtensionController.ensureBuiltIn for installing built-in extensions.

I've described the required steps here: #6356 (comment)

This is best reviewed by collapsing the tests. The actual changeset is pretty small and cleaned up the engine logic nicely (net negative lines of code). 🎉

@Amejia481 @psymoon Now that we've branched for release we can land this:

• The GV API landed in beta as well in the meantime, which means we can also remove the supportActions and allowContentMessaging fields (this is controlled by the manifest now)
• I will apply the change to beta once reviewed to make this easier to read
• The new GV API only installs if a newer version is available, solving the concerns described here: Install built-in extensions using GV extension controller #6356 (comment)
• To make sure the latest version is installed in development and in releases and to make sure we don't forget updating the version, we have: Closes #7278: Add task to generate versions for built-in extensions #7541

Marking as "Do not land" so I can coordinate landing with the required Fenix change.

[Amejia481]

🚀

[psymoon]

👍

[csadilek]

bors r=psymoon,Amejia481

[bors]

Build succeeded:

• complete-push