encoding/protojson, encoding/prototext: set wire unmarshal resolver

The text and JSON encodings for the google.protobuf.Any well-known type
require a call to proto.Unmarshal. Plumb through the resolver from the
UnmarshalOptions.

Change-Id: Iccc1a9d56acd9dd214f2b289216bd50acc2ef074
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/182980
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/encoding/protojson/encode.go b/encoding/protojson/encode.go
index 45d052c..fa28897 100644
--- a/encoding/protojson/encode.go
+++ b/encoding/protojson/encode.go
@@ -39,6 +39,7 @@
 	// Resolver is used for looking up types when expanding google.protobuf.Any
 	// messages. If nil, this defaults to using protoregistry.GlobalTypes.
 	Resolver interface {
+		protoregistry.ExtensionTypeResolver
 		protoregistry.MessageTypeResolver
 	}