Add attribute to optionally inline schemas when deriving

[michaeltlombardi]

As a developer and schema maintainer, I want to control which subschemas are inlined and which are placed in $defs.

I have a use-case where I want to define a struct that inlines a particular subschema but uses $defs for external schemas (creating a canonically bundled schema).

For example, I would like to be able to define something like the following (contrived) example:

mod tstoy {
    mod config {
        #[derive(Debug, Clone, PartialEq, Eq, Hash, Serialize, Deserialize, JsonSchema)]
        #[serde(rename_all = "camelCase")]
        pub enum Scope {
          Machine,
          User,
        }

        #[derive(Debug, Clone, PartialEq, Eq, Hash, Serialize, Deserialize, JsonSchema)]
        pub struct UpdateSettings {
            pub automatic: Option<bool>,
            pub frequency: Option<int>,
        }

        #[derive(Debug, Clone, PartialEq, Eq, Hash, Serialize, Deserialize, JsonSchema)]
        pub struct Instance {
            // This property should only be a $ref, no $defs
            #[serde(rename = "_exist", default)]
            #[schemars(schema_with = "dsc_rdk::standard_properties::reference_exist")]
            pub exist: bool,
            
            pub scope: ConfigScope,
            
            // Even though this property normally would be a $ref to an entry in $defs,
            // I want to inline the subschema without having to implement JsonSchema for
            // the struct.
            #[schemars(inline)]
            pub update: Option<UpdateSettings>,
        }
     }
  }

And have it emit something like the following (hand-authored) schema:

{
    "type": "object",
    "required": ["scope"]
    "properties": {
        "_exist": { "$ref": "https://schemas.example.com/v1/standard_properties/exist.json" },
        "scope": {
            "type": "string"
            "enum": ["machine", "user"]
        },
        "update": {
            "automatic": { "type": "boolean" }
            "frequency": { "type": "integer" }
        }
    },
    // I know that right now I'd have to insert this myself
    "$defs": {
        "https://schemas.example.com/v1/standard_properties/exist.json": {
            "type": "boolean,
            "default": true
        }
    }
}

I can probably figure out how to write appropriate transformers for this for my own usage, but I would prefer to be able to control whether a particular item in my struct is inlined without having to add extra custom handling.