PackWriter: make PackWriter.writeIndex() take a PackIndexWriter

PackWriter: make PackWriter.writeIndex() take a PackIndexWriter

Previously, the PackWriter implementation required that indexes and
their extensions be writable to an OutputStream with a fixed binary
format.  To support more general index storage formats, allow
PackWriter to accept an PackIndexWriter interface which accepts only
the objects to store. This allows implementors to choose their storage
format.

The implementation will be provided by the DfsObjectDatabase. The
DfsObjectDatabase is already responsible for providing the OutputStream
that was previously used to write indexes. Having it provide a writing
interface would be a natural generalization.

This idea was previously implemented for PackBitmapIndex writing in
https://gerrithub.io/c/eclipse-jgit/jgit/+/1177722.

Change-Id: I582d2f3d25d6adb2da243d6d0d7bc405a97d6183