lua: add helpers for encoding/decoding IPROTO packets

Closes #8054

@TarantoolBot document
Title: Document Lua helpers for encoding/decoding IPROTO packets

The following new constants and functions were added to the `box.iproto`
namespace:

 - `GREETING_SIZE`: Size of a Tarantool greeting message

 - `GREETING_PROTOCOL_LEN_MAX`: Max length of a protocol string that can
   be encoded in a Tarantool greeting message.

 - `GREETING_SALT_LEN_MAX`: Max length of a salt string that can be
   encoded in a Tarantool greeting message.

 - `box.iproto.encode_greeting({version = x, uuid = x, salt = x})`:
   Encodes a Tarantool greeting message. Takes a table. Returns a
   string. Raises on error. The protocol is set to "Binary" (IPROTO).

 - `box.iproto.decode_greeting(string)`: Decodes a Tarantool greeting
   message. Takes a string. Returns a table with the following fields:
   `protocol`, `version`, `uuid`, `salt`. Raises on error. The input
   string must be exactly `GREETING_SIZE` bytes long.

 - `box.iproto.encode_packet(header[, body])`: Encodes an IPROTO packet.
   Takes a packet header and optionally a body given as a table or a
   string. A table argument will be encoded in MsgPack. A string
   argument will be copied as is (it's supposed to contain valid MsgPack
   but it isn't enforced). Returns a string. Raises on error.

 - `box.iproto.decode_packet(string[, pos])`: Decodes an IPROTO packet.
   Takes a string containing one or more encoded IPROTO packets and
   optionally a position in the string to start decoding from. If the
   position is omitted, the function will start decoding from the
   beginning of the string. On success returns the decoded packet
   header, body, and the position in the string where decoding stopped.
   Both header and body are returned as `msgpack.object`. The body may
   be absent (set to nil). If the input is truncated, returns nil and
   the min number of bytes required to decode the packet. On failure,
   raises an error.

For examples, see [`test/app-luatest/iproto_encoder_test.lua`][1].

[1]: https://github.com/tarantool/tarantool/blob/master/test/app-luatest/iproto_encoder_test.lua