Add context to KV descriptor methods

[ondrej-fabry]

Add ctx context.Context as first parameter to KV descriptor methods and remove key string. Context provides propagation of request context starting from clients (grpc) and up to calls to remote services (govpp) helping to provide a lot of insight for developers.

Context in KV descriptor methods could be possibly used to:

• lookup info about the value:
  • object key (this could replace key parameter)
  • parent value (for derived values)
• lookup details about the current transaction (relevant TODO comment in link_state descriptor of VPP/ifplugin)
• make it possible to trace requests for their entire lifespan inside agent - drastically improving tracing capabilities
  (e.g. incoming gRPC req -> KVscheduler TX -> KVdescriptor CRUD -> SB impl (vppcalls..) -> 3rd-party )libs
• lookup info about:
  • client initiating the request?
  • object metadata?
  • source of data?
  • ... ?
• extend validation capabilities: e.g. check if IP address is already in use by other interfaces
• ... ?

[milanlenco]

Will start working on this in parallel with my main tasks since the need to have some context in descriptor just keeps reappearing :)