Handle unrecognized fields

[cthulhu-rider]

Unrecognized fields can be detected during unmarshaling of binary protobuf messages. These fields can carry sensitive data, so it seems that software should process them in a special way, such as denial of service or redirection to other parties.

Since almost all messages are signed/verified, unrecognized fields force verification failure even in case of correct signing. In this scenario sender will receive INCORRECT_SIGNATURE response status which is definitely unexpected.

I propose to think about possible reactions of applications on unrecognized fields and provide some functionality to catch/process them.