streaming: make the `odb_read_stream` definition public

streaming: make the `odb_read_stream` definition public

Subsequent commits will move the backend-specific logic of setting up an
object read stream into the specific subsystems. As the backends are now
the ones that are responsible for allocating the stream they'll need to
have the stream definition available to them.

Make the stream definition public to prepare for this.

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