fix: remove empty timestamp from the tx signature payload (gnolang#1939)

## Description

This PR closes gnolang#810 

I've dropped the unused `Timestamp` field from the `SignDoc`, which
forced all clients that generate tx signatures outside of the `gno` repo
to initialize it to an empty UTC timestamp value (ex. Adena,
`tm2-js-client`...)

**BREAKING CHANGE**: all clients that generate transaction signatures
without using the provided Go methods in this repository need to update
their source code to remove the `Timestamp` field. (@dongwon8247
@jefft0)

I've also provided error handling for generating these signature bytes,
which weren't present before.

<details><summary>Contributors' checklist...</summary>

- [x] Added new tests, or not needed, or not feasible
- [x] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [x] Updated the official documentation or not needed
- [ ] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [x] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>

gno.land/pkg/gnoclient/signer.go

@@ -96,7 +96,10 @@ func (s SignerFromKeybase) Sign(cfg SignCfg) (*std.Tx, error) {
 	}
 
 	// Derive sign doc bytes.
-	signbz := tx.GetSignBytes(chainID, accountNumber, sequenceNumber)
+	signbz, err := tx.GetSignBytes(chainID, accountNumber, sequenceNumber)
+	if err != nil {
+		return nil, fmt.Errorf("unable to get tx signature payload, %w", err)
+	}
 
 	sig, pub, err := s.Keybase.Sign(account, password, signbz)
 	if err != nil {

tm2/pkg/crypto/keys/client/sign.go

@@ -185,15 +185,21 @@ func signTx(
 	signOpts signOpts,
 	keyOpts keyOpts,
 ) error {
+	signBytes, err := tx.GetSignBytes(
+		signOpts.chainID,
+		signOpts.accountNumber,
+		signOpts.accountSequence,
+	)
+	if err != nil {
+		return fmt.Errorf("unable to get signature bytes, %w", err)
+	}
+
 	// Sign the transaction data
 	sig, pub, err := kb.Sign(
 		keyOpts.keyName,
 		keyOpts.decryptPass,
-		tx.GetSignBytes(
-			signOpts.chainID,
-			signOpts.accountNumber,
-			signOpts.accountSequence,
-		))
+		signBytes,
+	)
 	if err != nil {
 		return fmt.Errorf("unable to sign transaction bytes, %w", err)
 	}

tm2/pkg/crypto/keys/client/sign_test.go

@@ -444,7 +444,10 @@ func TestSign_SignTx(t *testing.T) {
 		require.NoError(t, err)
 
 		// Generate the signature
-		signature, pubKey, err := kb.Sign(anotherKey, encryptPassword, tx.GetSignBytes("id", 1, 0))
+		signBytes, err := tx.GetSignBytes("id", 1, 0)
+		require.NoError(t, err)
+
+		signature, pubKey, err := kb.Sign(anotherKey, encryptPassword, signBytes)
 		require.NoError(t, err)
 
 		tx.Signatures = []std.Signature{
@@ -554,7 +557,10 @@ func TestSign_SignTx(t *testing.T) {
 		require.NoError(t, err)
 
 		// Generate the signature
-		signature, pubKey, err := kb.Sign(keyName, encryptPassword, tx.GetSignBytes("id", 0, 0))
+		signBytes, err := tx.GetSignBytes("id", 0, 0)
+		require.NoError(t, err)
+
+		signature, pubKey, err := kb.Sign(keyName, encryptPassword, signBytes)
 		require.NoError(t, err)
 
 		tx.Signatures = []std.Signature{

tm2/pkg/sdk/auth/ante.go

@@ -153,7 +153,11 @@ func NewAnteHandler(ak AccountKeeper, bank BankKeeperI, sigGasConsumer Signature
 				// No signatures are needed for genesis.
 			} else {
 				// Check signature
-				signBytes := GetSignBytes(newCtx.ChainID(), tx, sacc, isGenesis)
+				signBytes, err := GetSignBytes(newCtx.ChainID(), tx, sacc, isGenesis)
+				if err != nil {
+					return newCtx, res, true
+				}
+
 				signerAccs[i], res = processSig(newCtx, sacc, stdSigs[i], signBytes, simulate, params, sigGasConsumer)
 				if !res.IsOK() {
 					return newCtx, res, true
@@ -430,15 +434,22 @@ func SetGasMeter(simulate bool, ctx sdk.Context, gasLimit int64) sdk.Context {
 
 // GetSignBytes returns a slice of bytes to sign over for a given transaction
 // and an account.
-func GetSignBytes(chainID string, tx std.Tx, acc std.Account, genesis bool) []byte {
+func GetSignBytes(chainID string, tx std.Tx, acc std.Account, genesis bool) ([]byte, error) {
 	var accNum uint64
 	if !genesis {
 		accNum = acc.GetAccountNumber()
 	}
-	signbz := std.SignBytes(
-		chainID, accNum, acc.GetSequence(), tx.Fee, tx.Msgs, tx.Memo,
+
+	return std.GetSignaturePayload(
+		std.SignDoc{
+			ChainID:       chainID,
+			AccountNumber: accNum,
+			Sequence:      acc.GetSequence(),
+			Fee:           tx.Fee,
+			Msgs:          tx.Msgs,
+			Memo:          tx.Memo,
+		},
 	)
-	return signbz
 }
 
 func abciResult(err error) sdk.Result {