rfc: consistent lua/sql types

There is discussion in #5910 about all inconsistencies
we see between Lua and SQL worlds and possible future
directions of SQL development and new types additions.

This RFC is current state of this discussion.

Part of #5910

Co-authored-by: Igor Munkin <imun@tarantool.org>
Co-authored-by: Peter Gulutzan <pgulutzan@ocelot.ca>
Co-authored-by: Mergen Imeev <imeevma@tarantool.org>
Co-authored-by: Sergey Ostanevich <sergos@tarantool.org>