packfile: expose function to read object stream for an offset

packfile: expose function to read object stream for an offset

The function `packfile_store_read_object_stream()` takes as input an
object ID and then constructs a `struct odb_read_stream` from it. In a
subsequent commit we'll want to create an object stream for a given
combination of packfile and offset though, which is not something that
can currently be done.

Extract a new function `packfile_read_object_stream()` that makes this
functionality available.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>