Add 'upgrader_pre_unpack' hook

[jdgrimes]

The WP_Upgrader class provides several hooks that can be used to modify the upgrader process. There is an 'upgrader_pre_download' hook, for example. But there is no hook that fires after download but before the package is unpacked. It would be nice to add such a hook, which for example could be used to verify a package's signature, or override the unpacking code entirely.

[NathanAtmoz]

Introduce upgrader_pre_unpack filter prior to unpacking the package.