introspection: format resulting schema from GraphQLClientSchema

[lrlna]

Description

To be able to display SDL ergonomically, we will need to make sure all types created by GraphQLClientSchema in #171 have a self.to_string method and Display, Debug trait implementations.

Detailed design

GraphQL Javascript reference implementation has specific printing requirements for each individual type, here is Scalar type, for example:

function printScalar(type: GraphQLScalarType, options): string {
  return (
    printDescription(options, type) +
    `scalar ${type.name}` +
    printSpecifiedByUrl(type)
  );
}

Rust implementation should then have something like this for it's Display trait impl:

impl fmt::Display for ScalarType {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        write!(fmt, "{}", self.to_string())
    }
}

ScalarType can then implement a to_string() that resembles its JavaScript counterpart outlined above.

This work is dependent on #171 to be completed first.

This issue is part of introspection work discussed in this internal scope document.