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
+
+class JSONDecoder: Foundation.JSONDecoder {
+  override init() {
+    super.init()
+  }
+
+  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)
+    } catch {
+      throw JSONDecodingError(error, data: data)
+    }
+  }
+
+  func convertNumberToString(
+    in input: Data,
+    at key: String
+  ) -> Data {
+    guard var string = String(data: input, encoding: .utf8) else {
+      return input
+    }
+    string = string.replacingOccurrences(
+      of: #""\#(key)"( *):( *)([0-9]+)( *)(,*)"#,
+      with: #""\#(key)"$1:$2"$3"$4$5"#,
+      options: [.regularExpression]
+    )
+    guard let output = string.data(using: .utf8) else {
+      return input
+    }
+    return output
+  }
+}
+
+public struct JSONDecodingError: Error, CustomStringConvertible, CustomDumpReflectable {
+  public init(_ underlayingError: Error, data: Data) {
+    self.underlayingError = underlayingError
+    self.data = data
+    self.string = String(data: data, encoding: .utf8)
+  }
+
+  public var underlayingError: Error
+  public var data: Data
+  public var string: String?
+
+  public var description: String {
+    var description = ""
+    customDump(self, to: &description)
+    return description
+  }
+
+  public var customDumpMirror: Mirror {
+    Mirror(
+      self,
+      children: [
+        "underlayingError": underlayingError,
+        "data": String(data: data, encoding: .utf8) ?? data
+      ],
+      displayStyle: .struct
+    )
+  }
+}