Expose more authentication packet to Lua

Currently proxy-plugin.c is failing to expose the authentication packet as a table to read_handshake.
Previously the code building the Lua table was directly in the proxy plugin.

To avoid code duplication and to be consistent with the rest of the exposed structures, there should be network-mysqld-packet-lua.(hc) files exposing this into the Lua state. That way the code can be reused in different plugins without copying code around.