box: introduce box_read_view_* ffi API functions

Introduce API for opening a read view over the specified spaces and indexes and creating iterators over the index read views.

NO_DOC=picodata patch

changelogs/unreleased/box_read_view.md

+## feature/box
+
+Introduce box_read_view_* ffi API for opening/closing read views and iterating
+over index read views.