refs: introduce `ref_store_init_options`

refs: introduce `ref_store_init_options`

Reference backends are initiated via the `init()` function. When
initiating the function, the backend is also provided flags which denote
the access levels of the initiator. Create a new structure
`ref_store_init_options` to house such options and move the access flags
to this structure.

This allows easier extension of providing further options to the
backends. In the following commit, we'll also provide config around
reflog creation to the backends via the same structure.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>