unknownKeys.swift

let json = """
{
    "foo": "bar",
    "baz": null
}
"""

struct Type: Decodable {
    let type: String
    let value: String?
}

struct Response: Decodable {
    let types: [Type]
    
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let dict = try container.decode([String: String?].self)
        
        types = try dict.map { element in
            let dict = ["type": element.key, "value": element.value]
            let data = try JSONEncoder().encode(dict)
            return try JSONDecoder().decode(Type.self, from: data)
        }
    }
}

let data = json.data(using: .utf8)!
do {
    let response = try JSONDecoder().decode(Response.self, from: data)
    print(response)
} catch {
    print(error)
}