Reject unknown fields in TxDecoder and sign mode handlers (#6883)

* WIP on unknown field rejection in TxDecoder

* WIP on unknown field rejection in TxDecoder

* WIP

* WIP

* WIP

* WIP

* Fix bugs with RejectUnknownFields

* Fix tests

* Fix bug and update docs

* Lint

* Add tests

* Add unknown field tests

* Lint

* Address review comments

client/tx/legacy_test.go

@@ -65,7 +65,7 @@ type TestSuite struct {
 func (s *TestSuite) SetupSuite() {
 	encCfg := simapp.MakeEncodingConfig()
 	s.encCfg = encCfg
-	s.protoCfg = tx.NewTxConfig(codec.NewProtoCodec(encCfg.InterfaceRegistry), std.DefaultPublicKeyCodec{}, tx.DefaultSignModeHandler())
+	s.protoCfg = tx.NewTxConfig(codec.NewProtoCodec(encCfg.InterfaceRegistry), std.DefaultPublicKeyCodec{}, tx.DefaultSignModes)
 	s.aminoCfg = types3.StdTxConfig{Cdc: encCfg.Amino}
 }