Generator: Lua: allow strings

[IamPete1]

This allows strings to be sent and received in AP with the generated parser.

Tested with:

local mavlink_msgs = require("MAVLink/mavlink_msgs")

local STATUSTEXT_ID = mavlink_msgs.get_msgid("STATUSTEXT")

local msg_map = {}
msg_map[STATUSTEXT_ID] = "STATUSTEXT"

-- initialise mavlink rx with number of messages, and buffer depth
mavlink:init(1, 10)

-- register message id to receive
mavlink:register_rx_msgid(STATUSTEXT_ID)

function update()
    local msg, chan = mavlink:receive_chan()
    if (msg ~= nil) then
        local parsed_msg = mavlink_msgs.decode(msg, msg_map)
        if (parsed_msg ~= nil) and (parsed_msg.msgid == STATUSTEXT_ID) then

            local ack = {}
            ack.severity = 0
            ack.text = "got msg: " .. parsed_msg.text

            mavlink:send_chan(chan, mavlink_msgs.encode("STATUSTEXT", ack))

        end
    end

    return update, 1000
end

return update()