Expose the PatchWriter generator in the public API.

We want to expose the patch writers constructors in the public API
without necesarily exposing the internal details. For that, we use a
factory in the public API so callers can create the PatchWriters they
need.

We will extend this file with new patch formats.

Bug: 34220646
Test: Used this interface from the bsdiff_main.cc (see follow up CL).
Change-Id: I1f005f103886579eacf37b7d9767b713e066030c