Allow introspection of default args for built-in scalars

[ozanmakes]

I made a quick attempt to add default values for arguments to the introspection query. Only covers built-in scalars for now.

I couldn't figure out if this information is already available in the introspection resolvers, so I added a new parameter to Arg.scalar. This will break backwards compatibility for people using custom scalar arguments, to avoid this I can make this argument optional. Please let me know if this is in the right direction.

Here's what it looks like in GraphiQL: