playermodels: Add option to disable resetting player color via ttt_enforce_playercolor convar

[Histalek]

This is mostly done to enhance compatibility with colourable
playermodels.

Before this change coloured playermodels would be set to COLOR_WHITE or
GAMEMODE.playercolor on (re)spawn to prevent players from being
distinguished by different colours.

After this change this behaviour is not changed unless ttt_enforce_playercolor is set to 0.
In that case we return early in the GM:TTTPlayerSetColor hook and do
nothing.
This does NOT however prevent other addons from hooking into the TTTPlayerSetColor hook nor
does it prevent other addons from setting the playercolor somewhere else.

NOTE: This is also not tested yet, hence draft status

[Histalek]

ttt_playercolor_mode = 0 seems to be unused [1]. Which looks like a better approach to this.

[1]

TTT2/gamemodes/terrortown/gamemode/shared/sh_main.lua

Lines 355 to 378 in fd45084

	---
	-- @realm shared
	-- stylua: ignore
	local colormode = CreateConVar("ttt_playercolor_mode", "1", {FCVAR_NOTIFY, FCVAR_ARCHIVE, FCVAR_REPLICATED})
	---
	-- @param string model The selected (default) playermodel
	-- @hook
	-- @realm shared
	function GM:TTTPlayerColor(model)
	local mode = colormode:GetInt()
	if mode == 1 then
	return ttt_playercolors.serious[math.random(ttt_playercolors_serious_count)]
	elseif mode == 2 then
	return ttt_playercolors.all[math.random(ttt_playercolors_all_count)]
	elseif mode == 3 then
	-- Full randomness
	return Color(math.random(0, 255), math.random(0, 255), math.random(0, 255))
	end
	-- No coloring
	return COLOR_WHITE
	end