snapuserd: Add an IBlockServer abstraction around dm-user.

This adds the planned interface that will abstract around dm-user (for
now), and later ublk at which point it can be adjusted as needed.

This declares the interface and implements it, but does not yet switch
snapuserd to use it. The implementation is copied from read_worker.cpp.

Bug: 288273605
Test: snapuserd_test
Change-Id: I060788c91dba78e52d315b5616b84b37eaf4040f