PSA New options for clone sessions

[jafingerhut]

From @samar-abdi. See p4lang/PI#353 for reference.

The requests I have heard in this area:

• Per clone session, add the option that instead of sending the clone to a single egress port, it is sent to a multicast group instead.

• Per clone session, add the option that the cloned packet is the one after ingress processing, not the pre-ingress-processing 'original' (I only put 'original' in quotes there because the 'original' is not necessarily the original one from an external input port, but could have been recirculated or resubmitted one or more times before becoming the packet at the beginning of this particular ingress step when the clone operation is invoked).