Avoid option conflicts between classs and packages

[lvjr]

Brief outline of the enhancement

From https://github.com/latex3/fontspec/issues/501, I think it would be great if LaTeX could provide some interfaces to reduce option conflicts between classes and packages using \DeclareKeys command.

Sometimes a class defines some key and hopes it to be used only by itself. So I think some key property .scope = local would be very useful:

\DeclareKeys {
   key .scope = local,
   key .code = {foo},
 }

And it is also good to provide a public command \RemoveKeyFromOptionsList.