Generalize auth/types.StdSignature (#4507)

New Signature interface available in the top level types package.
auth.StdSignature implements such interface. User defined auth
module can now define their own custom signature types.

Work carried out in the context of the following issues:
- #4488
- #4487

types/codec.go

@@ -6,4 +6,5 @@ import "github.com/cosmos/cosmos-sdk/codec"
 func RegisterCodec(cdc *codec.Codec) {
 	cdc.RegisterInterface((*Msg)(nil), nil)
 	cdc.RegisterInterface((*Tx)(nil), nil)
+	cdc.RegisterInterface((*Signature)(nil), nil)
 }

types/coin_test.go

@@ -4,9 +4,10 @@ import (
 	"strings"
 	"testing"
 
-	"github.com/cosmos/cosmos-sdk/codec"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+
+	"github.com/cosmos/cosmos-sdk/codec"
 )
 
 var (

types/tx_msg.go

@@ -2,6 +2,8 @@ package types
 
 import (
 	"encoding/json"
+
+	"github.com/tendermint/tendermint/crypto"
 )
 
 // Transactions messages must fulfill the Msg
@@ -42,6 +44,12 @@ type Tx interface {
 
 //__________________________________________________________
 
+// Signature defines the properties of the signature payload.
+type Signature interface {
+	GetPubKey() crypto.PubKey
+	GetSignature() []byte
+}
+
 // TxDecoder unmarshals transaction bytes
 type TxDecoder func(txBytes []byte) (Tx, Error)