feat: Control API uses automatic cross-pinning mTLS (Closes #119)

. kapow server generates on startup a pair of certificates
that will use to secure communications to its control server.
It will communicate the server and client certificates as well
as the client private key to the init programs it launches,
via environment variables.

. kapow server now understands a new flag --control-reachable-addr
which accepts either a IP address or a DNS name, that can be used
to ensure that the generated server certificate will be appropiate
in case the control server must be accessed from something other
than localhost.

Co-authored-by: Roberto Abdelkader Martínez Pérez <robertomartinezp@gmail.com>

internal/server/server.go

@@ -19,6 +19,7 @@ package server
 import (
 	"sync"
 
+	"github.com/BBVA/kapow/internal/certs"
 	"github.com/BBVA/kapow/internal/server/control"
 	"github.com/BBVA/kapow/internal/server/data"
 	"github.com/BBVA/kapow/internal/server/user"
@@ -34,13 +35,16 @@ type ServerConfig struct {
 
 	ClientAuth,
 	Debug bool
+
+	ControlServerCert certs.Cert
+	ControlClientCert certs.Cert
 }
 
 // StartServer Starts one instance of each server in a goroutine and remains listening on a channel for trace events generated by them
 func StartServer(config ServerConfig) {
 	var wg = sync.WaitGroup{}
 	wg.Add(3)
-	go control.Run(config.ControlBindAddr, &wg)
+	go control.Run(config.ControlBindAddr, &wg, config.ControlServerCert, config.ControlClientCert)
 	go data.Run(config.DataBindAddr, &wg)
 	go user.Run(config.UserBindAddr, &wg, config.CertFile, config.KeyFile, config.ClientCaFile, config.ClientAuth, config.Debug)