libsnapshot: Implement CowWriterV3::EmitzeroBlocks.

Add zero block operation to writer v3. We currently write this operation
to after the scratch space. Once resume point support is added we may
need to change this to write after the resume point. This CL only
supports zero blocks, so writeoperation does not take in data yet.

Bug: 307452468
Test: cow_api_test
Change-Id: I659b2e2e4f6e0d96e374ed29012318cc34b4158d