Log reverse correlation id

Closes #68 (closed).

This uses gitlab-org/labkit!117 (merged) and gitlab-org/labkit!119 (merged) to send correlation id from the server and to get it on the client side. Then the id is attached to an error, which then is logged via a custom field constructor for zap.

Logs from kas:

{"level":"error","time":"2021-06-30T14:57:22.252+1000","msg":"GetAgentInfo()","correlation_id":"01F9DMER27BAA2YYHK0XZ7R6J7","grpc_service":"gitlab.agent.reverse_tunnel.rpc.ReverseTunnel","grpc_method":"Connect","error":"Get \"http://gdk.test:3000/api/v4/internal/kubernetes/agent_info\": dial tcp 172.16.123.1:3000: connect: connection refused"}

Logs from agent:

{"level":"error","time":"2021-06-30T14:57:22.252+1000","msg":"Reverse tunnel","mod_name":"reverse_tunnel","error":"rpc error: code = Unavailable desc = unavailable","correlation_id":"01F9DMER27BAA2YYHK0XZ7R6J7"}