Added ShowMenu implementation

[vinci6k]

As promised: https://forums.sourcepython.com/viewtopic.php?f=37&t=2472#p15210

Tested in CSS and CSGO with:

# ../showmenu_hook/showmenu_hook.py

# Source.Python
from messages.hooks import HookUserMessage


@HookUserMessage('ShowMenu')
def showmenu_pre(recipients, buffer):
    print(recipients, buffer)

[Ayuto]

Thank you! Have you check very big menus on CS:S (bitbuffer)? Because there should actually be a byte that tells us whether or not the menu is chunked (right before the menu string):
https://github.com/Source-Python-Dev-Team/Source.Python/blob/master/addons/source-python/packages/source-python/messages/base.py#L242-L263

[vinci6k]

I used change_map.py to test the hook and oddly enough, nothing seemed wrong. 😅