Add CVar attribute to register on value changed automatically for fields, properties, and methods on systems, services, and controllers

[DrSmugleaf]

Example usage:

[CVar<CVars>(nameof(CVars.NetMaxConnections))]
public int Value;

[CVar<CVars>(nameof(CVars.NetMaxConnections))]
public void OnMaxConnectionsChanged(int val)
{
    ...
}

Now I don't need to remind every contributor to pass true to OnValueChanged forget 1 time and be hit by the curse of Ra