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

codec/unknownproto/doc.go

@@ -6,22 +6,18 @@ a) Unknown fields in the stream -- this is indicative of mismatched services, pe
 
 b) Mismatched wire types for a field -- this is indicative of mismatched services
 
-Its API signature is similar to proto.Unmarshal([]byte, proto.Message) as
+Its API signature is similar to proto.Unmarshal([]byte, proto.Message) in the strict case
 
-    ckr := new(unknownproto.Checker)
-    if err := ckr.RejectUnknownFields(protoBlob, protoMessage); err != nil {
+    if err := RejectUnknownFieldsStrict(protoBlob, protoMessage, false); err != nil {
             // Handle the error.
     }
 
 and ideally should be added before invoking proto.Unmarshal, if you'd like to enforce the features mentioned above.
 
 By default, for security we report every single field that's unknown, whether a non-critical field or not. To customize
-this behavior, please create a Checker and set the AllowUnknownNonCriticals to true, for example:
+this behavior, please set the boolean parameter allowUnknownNonCriticals to true to RejectUnknownFields:
 
-    ckr := &unknownproto.Checker{
-            AllowUnknownNonCriticals: true,
-    }
-    if err := ckr.RejectUnknownFields(protoBlob, protoMessage); err != nil {
+    if err := RejectUnknownFields(protoBlob, protoMessage, true); err != nil {
             // Handle the error.
     }
 */