Initial prototype of COW format and API.

This is an initial prototype of the new COW format. It does not have
support for merge sequencing or merge state tracking.

The reader and writer APIs have pure virtual interfaces to support
mocking. The writer implementation performs compression, but the reader
interface does not (yet), it only supports iterating over metadata and
performing basic validation.

Bug: 162274240
Test: manual tests
      cow_api_test gtest
Change-Id: Ib9fddc1f210700688e6786917b023dfabd5fb3d9