Allow defining custom ruleset options

[XHawk87]

Is your feature request related to a problem? Please describe.
Currently, server options are hard-coded in C++.

freeciv21/server/settings.cpp

Lines 1337 to 1338 in 25ce4fe

	// game settings
	static struct setting settings[] = {

I'd like to be able to define additional options to be used in lua scripts so that game admins can manipulate them using the familiar /set command.

Describe the solution you'd like
Add a new server.setting.create command in lua, and possibly also server.setting.set.

Describe alternatives you've considered
Move all settings over to default.lua and add a new optional settings.lua file, so that settings can be defined and set more flexibly than just using the /set command and [settings] section of game.ruleset.

Additional context
Relating to this discussion on discord