Instead of simply taking the oid type, future-proof our index opening
and creation functionality by taking an options structure.

Object ID type should be an option within the options structure; move it
there.

odb wrapping

The `wrap_odb` function doesn't need to know the OID types in the object
database; the object database already knows the type.

Future-proof the SHA256-ification of `git_repository_new` by taking an
options structure instead of an oid type.

Instead of making the commit and dump functions take individual options
structures; provide the options structure to the writer creator. This
allows us to add additional information (like OID type) during
generation.

Provide an options structure to commit graph opening. This allows us to
specify information (like OID type) during opening.

Provide an options structure to MIDX writing. This allows us to
specify information (like OID type) during writer creation.