Conditionally allow introspection

[pavelnikolov]

This PR adds a schema option which conditionally allows introspection based on a filter function. For example the below logic would be possible:

filter := func (ctx context.Context) bool {
    u, ok := user.FromContext(ctx)
    return ok && u.IsAdmin()
}
graphql.RestrictIntrospection(filter)

fixes: #556