Cryptobox serializer should be configurable

[oberstet]

WAMP-cryptobox should allow for different serializers to be used when serializing the application payload that is to be end-to-end encrypted. See here and here.

The hooks to allow use of a configurable serializer are there, but it is hard-coded to "json" currently.

We should support all serializers that we support at the transport level.

Another question is what default cryptobox serializer to use when none was specified. And also how to let the user configure the cryptobox serializer if it shouldn't use the default one.