Add SkipSequenceCheck flag in SignatureV2 for Amino signatures (#7234)

* Add SkipSequenceCheck

* Fix test

* Fix test

* Add explicit amino test

Co-authored-by: Alexander Bezobchuk <alexanderbez@users.noreply.github.com>

types/tx/signing/signature.go

@@ -19,8 +19,23 @@ type SignatureV2 struct {
 	// the signatures themselves for either single or multi-signatures.
 	Data SignatureData
 
-	// Sequence is the sequence of this account.
+	// Sequence is the sequence of this account. Only populated in
+	// SIGN_MODE_DIRECT.
 	Sequence uint64
+
+	// Ugly flag to keep backwards-compatibility with Amino StdSignatures.
+	// In SIGN_MODE_DIRECT, sequence is in AuthInfo, and will thus be populated
+	// in the Sequence field above. The ante handler then checks this Sequence
+	// with the actual sequence on-chain.
+	// In SIGN_MODE_LEGACY_AMINO_JSON, sequence is signed via StdSignDoc, and
+	// checked during signature verification. It's not populated in the
+	// Sequence field above. This flag indicates that the Sequence field should
+	// be skipped in ante handlers.
+	// TLDR;
+	// - false (by default) in SIGN_MODE_DIRECT
+	// - true in SIGN_MODE_LEGACY_AMINO_JSON
+	// ref: https://github.com/cosmos/cosmos-sdk/issues/7229
+	SkipSequenceCheck bool
 }
 
 // SignatureDataToProto converts a SignatureData to SignatureDescriptor_Data.