Add helpers for encoding and decoding IPROTO requests

[locker]

In the scope of #7716, we added the ability to override specific IPROTO requests on a configured Tarantool sever and send IPROTO responses over an IPROTO connection established by the server. Unfortunately, there's still no convenient helpers available that would allow to implement the IPROTO protocol in Lua on top of raw sockets, without configuring box. It could be useful for application developers that want to emulate IPROTO for some reason as well as in tests. Currently, we have to generate IPROTO requests manually, which is tedious:

tarantool/test/box-luatest/ghs_16_user_enumeration_test.lua

Lines 18 to 43 in 2574ff1

	local function auth(sock_path, user, tuple)
	local hdr = msgpack.encode({[IPROTO_REQUEST_TYPE] = IPROTO_AUTH})
	local body = msgpack.encode({
	[IPROTO_USER] = user,
	[IPROTO_TUPLE] = tuple,
	})
	local len = hdr:len() + body:len()
	t.assert_lt(len, 256)
	local s = socket.tcp_connect('unix/', sock_path)
	local data = s:read(128) -- greeting
	t.assert_equals(#data, 128)
	data = '\xce\00\00\00' .. string.char(len) .. hdr .. body
	t.assert_equals(s:write(data), #data) -- request
	data = s:read(5) -- fixheader
	t.assert_equals(#data, 5)
	len = msgpack.decode(data)
	data = s:read(len) -- response
	t.assert_equals(#data, len)
	s:close()
	hdr, len = msgpack.decode(data)
	body = msgpack.decode(data, len)
	return {
	bit.band(hdr[IPROTO_REQUEST_TYPE], bit.bnot(IPROTO_TYPE_ERROR)),
	body[IPROTO_ERROR],
	}
	end

What we need is:

• Encoding and decoding of a greeting message.
• Generating authentication data.
• Handling of request/response fix header.

This is all done by net.box under the hood. We could probably just make the code public.