Create a more idiomatic way of building a SEP010 Challenge Transaction

[ryanleecode]

Creating a challenge transaction for the SEP010 protocol should be something thats embeded in the SDK, given that most of the parameters can be predetermined.

Current to build such a transaction, you would need to do this...

	randomNounce, err := GenerateRandomString(48)
	if err != nil {
		panic(err)
	}

	var seed [32]byte
	copy(seed[:], []byte("MY_SECRET_KEY_HERE"))
	fiKeyPair, err := keypair.FromRawSeed(seed)
	if err != nil {
		panic(err)
	}
	account.Sequence = "0"
	currentTime := time.Now().UTC().Unix()
	tx := txnbuild.Transaction{
		SourceAccount: &account,
		Timebounds: txnbuild.NewTimebounds(
			currentTime,
			currentTime + 300,
		),
		Operations: []txnbuild.Operation{
			&txnbuild.ManageData{
				SourceAccount: &account,
				Name: "Stellar FI Anchor auth",
				Value: []byte(randomNounce),
			},
		},
		Network: network.TestNetworkPassphrase,
	}
	base64Txn, err := tx.BuildSignEncode(fiKeyPair)

Note that I have set the account sequence to "0" but the transaction builder will increment it to 1. And you can't set it to -1 because you will get
couldn't build transaction: failed to parse sequence number: Failed to parse account sequence number: strconv.ParseUint: parsing "-1": invalid syntax.

Generating a random nounce is trival but still an annoyance.

Something like the following would be a lot less effort.

	var seed [32]byte
	copy(seed[:], []byte("MY_SECRET_KEY_HERE"))
	fiKeyPair, err := keypair.FromRawSeed(seed)
	if err != nil {
		panic(err)
	}
	tx := txnbuild.Transaction{
		SourceAccount: &account,
		Network: network.TestNetworkPassphrase,
	}
        anchorName := "Stellar FI Anchor auth"
        tx.BuildChallenge(anchorName)
        tx.Sign(fiKeyPair)
	base64Txn, err := tx.Base64()

[ire-and-curses]

Yes, we need to address at a minimum the "0" problem ASAP, but I really like this convenience function idea as well. @poliha let's hold off on this week's planned release until we can include something to address this, and get it in at the same time. Anchors are implementing SEP 10 so this is urgent.

[bartekn]

Note that I have set the account sequence to "0" but the transaction builder will increment it to 1.

This is tracked in #1259.

[poliha]

There is already a workaround for the "0" problem. Using Transaction.SimpleAccount struct you can set Sequence to -1. We do something similar for the compliance server, see here. This should help before we implement the convenience function.

[ire-and-curses]

@poliha is right. For this edge case (and in general when creating accounts manually), you can create a SimpleAccount. It has the following shape:

type SimpleAccount struct {
	AccountID string
	Sequence  int64
}

Here Sequence is an int64 so can be set to -1 and incremented to 0 in the build step as you needed. The convenience function is still nice but less urgent.

[poliha]

Closed in #1468