refs: add a backend method structure

Add a `struct ref_storage_be` to represent types of reference stores. In
OO notation, this is the class, and will soon hold some class
methods (e.g., a factory to create new ref_store instances) and will
also serve as the vtable for ref_store instances of that type.

As yet, the backends cannot do anything.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.h

@@ -544,4 +544,6 @@ int reflog_expire(const char *refname, const unsigned char *sha1,
 		  reflog_expiry_cleanup_fn cleanup_fn,
 		  void *policy_cb_data);
 
+int ref_storage_backend_exists(const char *name);
+
 #endif /* REFS_H */