Custom JSONEncoder & JSONDecoder

This MR adds custom JSONEncoder and JSONDecoder that handles non-standard JSON values used by the go client.

• Handle BigInt values in JSON at key Value by converting them to/from String.
• Add JSONEncodingError and JSONDecodingError for extra verbose output when debugging JSON coding-related issues.

The JSONEncoder and JSONDecoder are internal, to avoid name collisions when using the XXClient library as a dependency, but internally all models are encoded/decoded using the custom encoder/decoder. Convenient encode and decode methods on each model remain public.

Sources/XXClient/Helpers/JSONDecoder.swift

 import CustomDump
 import Foundation
 
-public class JSONDecoder: Foundation.JSONDecoder {
-  public override init() {
+class JSONDecoder: Foundation.JSONDecoder {
+  override init() {
     super.init()
   }
 
-  public override func decode<T>(_ type: T.Type, from data: Data) throws -> T where T: Decodable {
+  override func decode<T>(_ type: T.Type, from data: Data) throws -> T where T: Decodable {
     do {
       let data = convertNumberToString(in: data, at: "Value")
       return try super.decode(type, from: data)