VXLAN-GPE transport for In-band OAM

• Use VxLAN-GPE to carry extension headers containing iOAM TLVs.
• As outlined in [draft-brockners-inband-oam-transport-00], in-band OAM data records can be transported in VXLAN-GPE, tunnels. [draft-brockners-inband-oam-data] further describes these TLVs.
• As part of the code changes, the forwarding code is restructured to separate the transport-agnostic and transport-specific portions.
• Three different scenarios need to be handled by the code:

1. VxLAN encap : Add iOAM headers as part of the VxLAN encap functionality.
2. VxLAN decap and preparation of export: Consume information received in iOAM TLVs as part of the VxLAN tunnel termination. The consumed information can be exported as IPFIX records to an out-of-band collector for analysis/ analytics.
3. VxLAN transit: This case requires some DPI to populate iOAM TLVs in a VxLAN underlay (transit) node.

• Please note that, while the above problem description is provided for completeness, commits could be phased for logistical reasons.