[#52] feat(Vfs): support compressed VDF Union files

[skejt23]

This PR adds support for reading and saving zipped VDF files (Union ZippedStream format).
The support is optional (disabled by default) and can be turned on with ZK_ENABLE_ZIPPED_VDF cmake option.

See the issue #52 for more details.

What's been done to verify that it works:

• Verified that Union.vdf file mentioned in the issue above can be successfully loaded by ZenKit.
• Verified that "VDFSTool" (made by one of the authors of Union) which is now commonly used instead of "GothicVDFS" can read zipped files created by ZenKit, and correctly qualifies them as "zipped".
• Added two test cases: one which reads basic_zipped.vdf file that was saved as zipped by the aforementioned VDFSTool, and the second which tests the round-trip: saves vdf as zipped, and then reads it.