Workaround for big numbers in JSONs

This MR introduces a workaround for an issue where JSON models containing big numbers (eg 930-digits) can't be parsed by swift's JSONDecoder/JSONEncoder.

This affects the GroupMember model, which holds DHKey models that has value property with such a big number. The model's value is stored as String, and it's automatically decoded/encoded to big number in JSON.

Sources/XXClient/Helpers/JSONHelpers.swift

 import Foundation
 
+/// Replaces all numbers at provided key with string equivalents
+///
+/// Example input:
+/// {
+///   "key": 123,
+///   "object": {
+///     "hello": "world",
+///     "key": 321
+///   }
+/// }
+///
+/// Example output:
+/// {
+///   "key": "123",
+///   "object": {
+///     "hello": "world",
+///     "key": "321"
+///   }
+/// }
+///
+/// - Parameters:
+///   - input: JSON data
+///   - key: the key which values should be converted
+/// - Returns: JSON data
 func convertJsonNumberToString(
   in input: Data,
   at key: String
@@ -18,6 +42,30 @@ func convertJsonNumberToString(
   return output
 }
 
+/// Replaces all strings at provided key with number equivalents
+///
+/// Example input:
+/// {
+///   "key": "123",
+///   "object": {
+///     "hello": "world",
+///     "key": "321"
+///   }
+/// }
+///
+/// Example output:
+/// {
+///   "key": 123,
+///   "object": {
+///     "hello": "world",
+///     "key": 321
+///   }
+/// }
+///
+/// - Parameters:
+///   - input: JSON data
+///   - key: the key which values should be converted
+/// - Returns: JSON data
 func convertJsonStringToNumber(
   in input: Data,
   at key: String