Use block string syntax in serialization

[dariuszkuc]

Description

GraphQL supports both single line (wrapped in ") and multiline comments (wrapped in """). apollo-rs currently does not support multiline comments and coalesce it to a single line, i.e.

"""
First line comment
Second line comment
"""

becomes

"First line comment\nSecond line comment"

Steps to reproduce

#[test]
    fn comments() {
        let raw_comment = r#""""
First Line
Second Line
""""#;
        let multiline_comment = "First Line
Second Line";

        let foo = ObjectType {
            name: Name::new("foo"),
            description: Some(NodeStr::new(multiline_comment)),
            directives: vec![],
            fields: IndexMap::new(),
            implements_interfaces: IndexMap::new(),
        };

        let mut schema = Schema::new();
        schema
            .types
            .insert(Name::new("foo"), ExtendedType::Object(Node::new(foo)));
        println!("{}", schema);
    }

Expected result

Printed descriptions should preserve multiline comments.

Actual result

When using raw_string (from above) prints following schema

"\"\"\"\nFirst Line\nSecond Line\n\"\"\""
type foo

When using multiline_comment (from above) prints following schema

"First Line\nSecond Line"
type foo

Environment

• Operating system and version: MacOS Ventura
• Shell (bash/zsh/powershell): zsh
• apollo-rs crate: mut branch
• Crate version: 1.72.0

[SimonSapin]

Implementation PR: #724

See also stylistic question: #724 (comment)