VPP compatibility

In vpp-agent, we maintain compatibility with multiple versions of VPP using a single branch.

we need to do following for each VPP version:

• generate Go code for the VPP binary API using GoVPP's binapi-generator
• implement VPP core API and other VPP handler APIs (vppcalls packages) using the generated binapi
• register the handler implementation together along binapi messages used
• include a blank import for the implemented handler package to include in the build

To achieve multi-version support in the same build we need to pick the right implementation during runtime. Upon connecting to VPP we do a compatibility check by checking a list of binapi message. We try each include version one by one until we find one for which all messages pass the compatibility check.

VPP handlers