Better support of encapsulation

[lspgn]

This is an enhancement idea.
Provide a better support of encapsulated packets.
This concerns sFlow and IPFIX frame/datagrams fields.

For instance:

Packet
Ethernet
IP
IP
TCP
Payload

Or more complex:

Packet
Ethernet
IP
UDP (GRE)
Ethernet
IP
TCP
Payload

Are currently not fully decoded. The secondary layers are omitted.

One idea would to only support custom mapping for encapsulation.
This would require to expose protocol numbers (from IP next header) and IEEE 802 numbers (GRE, MPLS...) to custom mapping parsers.