Extend pkg with delegate/redelegate/unbond and withdraw

[MarcelMWS]

extend pkg

[pasqenr]

I have some concerns about this PR in its current status.

1. We must wait that @MarcelMWS update his repository with the latest commits merged recently ;
2. The fields in the new classes use a snake_case naming (e.g. delegator_address) but the Dart naming style suggest the lowerCamelCase coding style for identifiers;
3. The new classes extend StdMsg and override the getter of the field value, e.g.

@override
Map<String, dynamic> get value => {
  'delegator_address': this.delegator_address,
  'validator_src_address': this.validator_src_address,
  'validator_dst_address': this.validator_dst_address,
  'amount': this.amount.map((coin) => coin.toJson()).toList(),
};

I think that a better way of doing this is to use directly the constructor:

MsgDelegate({
  @required this.delegator_address,
  @required this.validator_address,
  @required this.amount,
  })  : assert(delegator_address != null),
    assert(validator_address != null),
    assert(amount != null),
    super(
      type: "cosmos-sdk/MsgDelegate",
        value: {
          'delegator_address': delegator_address,
          'validator_address': validator_address,
          'amount': amount.map((coin) => coin.toJson()).toList(),
        },
    );

The only issue can be the immutability of the StdMsg field value because currently it's non-final. @marcotradenet maybe we should consider making it final directly in the StdMsg superclass (final Map<String, dynamic> value).

4. Maybe we should re-format the json files in test_resources directory, VSCode format command should be enough.

[MarcelMWS]

Hi thanks for the review. I will fix this as soon as possible