This Pull Request introduces improvements to the JSON schema generation process, specifically for Map types. These improvements are in line with the JSON schema draft documentation and the output from the schemars tool.

When a Map is defined, it generates a JSON schema definition where additionalProperties is set to accept the type required by the map value type.

Consider the following Rust code:

#[cw_serde]
#[derive(QueryResponses)]
pub enum QueryMsg {
    /// # MapString
    #[returns(BarResponse)]
    MapString { foo: BTreeMap<String, String> },
    #[returns(BarResponse)]
    MapWithValue { foo: BTreeMap<String, Value> },
}

This code generates the following JSON schemas:

"map_string": {
    "type": "object",
    "required": ["foo"],
    "properties": {
        "foo": {
            "type": "object",
            "additionalProperties": {
                "type": "string"
            }
        }
    },
    "additionalProperties": false
}

"map_with_value": {
    "type": "object",
    "required": ["foo"],
    "properties": {
        "foo": {
            "type": "object",
            "additionalProperties": {
                "$ref": "#/definitions/Value"
            }
        }
    },
    "additionalProperties": false
}

In Go, this should generate a foo property of type map[string]string for the first schema and map[string]Value for the second one.

type QueryMsg_MapString struct {
	Foo map[string]string `json:"foo"`
}

type QueryMsg_MapWithValue struct {
	Foo map[string]Value `json:"foo"`
}

This PR includes modifications to the handling of the additionalProperties field, allowing it to accept either a boolean or a JSONSchema, in line with the JSON schema specification.

Furthermore, when additionalProperties is set to true (which is the default value in JSON Schema), it implies that the object can accept any property with any value. While this does not adhere strictly to the JSON schema specification (because we can set both properties and addionalProperties), it provides a more flexible approach to handling additionalProperties.