pairs/ipairs metamethods should work with userdata

[CodeSmile-0000011110110111]

For a LuaList type I was trying to implement the Metamethod.IPairs and Pairs to enumerate the values in the list from within Lua using the standard pairs/ipairs functions.

However this fails:

Lua.LuaRuntimeException : Lua-CSharp: .../LuaListTests.lua:22: bad argument #1 to 'pairs' (table expected, got userdata))

The default Pairs and IPairs functions both read a table as the first argument:

    public async ValueTask<int> Pairs(LuaFunctionExecutionContext context, CancellationToken cancellationToken)
    {
        var arg0 = context.GetArgument<LuaTable>(0);

This limits the use of Metamethod.Pairs and IPairs. It should work with both LuaTable and ILuaUserData arguments.
According to this user Lua 5.2 does support pairs/ipairs to work with userdata.

On first sight it might work by changing to GetArgument<ILuaUserData> and then testing for whichever interface implements TryGetValue(key, value). Not sure if this will require ILuaUserData to define that. It will also require LuaTable to implement ILuaUserData (it already does anyway).