MessageHeader.fromJson constructor
Factory method to create a MessageHeader instance from JSON.
Implementation
factory MessageHeader.fromJson(Map<String, dynamic> json) {
  return MessageHeader(
    numRequiredSignatures: json['numRequiredSignatures'],
    numReadonlySignedAccounts: json['numReadonlySignedAccounts'],
    numReadonlyUnsignedAccounts: json['numReadonlyUnsignedAccounts'],
  );
}